One might think that since the vertebrates had a better-developed nervous system than do any other group of creatures, they would naturally have the fastest-moving nerve impulses and therefore the thickest nerve fibers. This is not so. The lowly cockroach has some axons thicker than any in man. The extreme, as far as sheer size is concerned, is reached in the largest and most advanced mollusks, the squids. The large squids probably represent the most advanced and highly organized of all the invertebrates. Combining that with their physical size, we find it not surprising that they require the fastest nerve-impulse rates and the thickest axons. Those leading to the squid's muscles are

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commonly referred to as "giant axons" and are up to a millimeter in diameter. These are 50 times the diameter of the thickest mammalian axons and possess 2500 times the cross-sectional area. The giant axons of the squid have proved a godsend to neurologists, who were able to perform experiments on them easily (such as measuring directly the electrical potential across the axon membrane) that could be performed only with great difficulty upon the tenuous fibers present in vertebrates.

And yet why should invertebrates outpace vertebrates in axon thickness when the vertebrates have the more highly developed nervous system? The answer is that vertebrates do not depend on thickness alone. They have developed another and more subtle method for increasing the rate of propagation of the nerve impulse.

In vertebrates, the nerve fiber is surrounded during its process of formation in very earlv life with "satellite cells." Some of these are referred to as Schwann's cells (shvahnz) after the German zoologist Theodor Schwann, who was one of the founders of the cell theory of life. The Schwann's cells fold themselves about the axon, wrapping in a tighter and tighter spiral, coating the fiber with a fatty layer called the myelin sheath (my'uh-h'n; "marrow" G, the reason for the name being uncertain). The Schwann's cells finally form a thin membrane called the neurilemma (nyoo'rih-lem'uh; "nerve-skin" G), which still contains the nuclei of the original Schwann's cells. (It was Schwann who first described the neurilemma in 1839, so that it is sometimes called the "sheath of Schwann." As a result a rather unmusical and unpleasant memorial to the zoologist lies in the fact that a tumor of the neurilemma is sometimes called a "schwannoma.")

A particular Schwann's cell wraps itself about only a limited section of the axon. As a result, an axon has a myelin sheath that exists in sections. In the intervals betweer the original Schwann's cells are narrow regions where the myelin sheath does not exist, so that the axon rather resembles a string of sausages. These constricted unmyelinated regions are called nodes of Ran-vier (rahn-vee-ay), after the French histologist Louis Antoine

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Ranvier, who described them in 1878. The axon is like a thin line running down the axis of the interrupted cylinder formed by the myelin sheath. The word "axon" comes from "axis," in fact, with the "on** suffix substituted because of the presence of that suffix in the word "neuron."

The functions of the myeh'n sheath are not entirely clear to us. The simplest suggestion is that they insulate the nerve fiber, allowing a smaller loss of electric potential to the surroundings. Such a loss increases as a nerve fiber grows thinner, so that the presence of insulation allows a fiber to remain thin without undue loss of efficiency. Evidence in favor of this rests in the fact that the myelin sheath is composed largely of fatty material, which is indeed a good electrical insulator. (It is this fatty material that gives nerves their white appearance. The cell body of the nerve is grayish.)

If the myelin sheath served only as an insulator and nothing else, very simple fat molecules would do for the purpose. Instead, the chemical composition of the sheath is most complex. About two out of every five molecules within it are cholesterol. Two more are phosphatides (a type of phosphorus-containing fat molecule), and a fifth is a cerebroside (a complex sugar-containing fatlike molecule). Small quantities of other unusual substances are present, too. The existence of sheath properties other than mere insulation would therefore seem very likely.

It has also been suggested that the myelin sheath serves somehow to maintain the integrity of the axon. The axon stretches so far from the cell body that it seems quite reasonable to assume it can no longer maintain active communication throughout its length with the cell nucleus — and the nucleus is vital to cellular activity and integrity. Perhaps the sheath cells, which retain their nuclei, act as nursemaids, in a manner of speaking, for successive sections of the axon. Even nerves without myelin sheaths have axons that are surrounded by layers of small Schwann's cells with, of course, nuclei.

Finally, the sheath must  somehow accelerate the  speed  of

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propagation of the nerve impulse. A fiber with a sheath conducts the impulse much more rapidly than a fiber of the same diameter without a sheath. That is why vertebrates have been able to retain thin fibers and yet vastly increase the speed of impulse.

In the mammalian myelinated nerve, the impulse travels at a velocity of about 100 meters per second, or, if you prefer, 225 miles an hour. This is quite fast enough. The greatest stretch of mammalian tissue that has ever existed in a single organism is the hundred feet separating the nose end of a blue whale from its tail end; tbe nerve impulse along a myelinated axon could negotiate that distance in 3/10 of a second. The distance from head to toe in a man six feet tall could be covered in 1/50 of a second. The superiority of the nerve impulse, in terms of speed, over hormone coordination is, as you see, tremendous.

The process of myelination is not quite complete at birth, and various functions do not develop until the corresponding nerves are myelinated. Thus, the baby does not see at first; this must await the myelinization of the optic nerve, which, to be sure, is not long delayed. Also, the nerves connecting to the muscles of the legs are not fully myelinated until the baby is more than a year old, and the complex interplay of muscles involved in walking are delayed that long.

Occasionally an adult will suffer from a "demyelinating disease" in which sections of myelin sheathing degenerates, with consequent loss of function of tbe nerve fiber. The best known of these diseases is multiple sclerosis ("hard" G). This receives its name because demyelination occurs in many patches, spread out over the body, the soft myelin being replaced by a scar of harder fibrous tissue. Such demyelination may come about through the effect on the myelin of a protein in the patient's blood. The protein seems to be an "antibody," one of the class of substances that usually react only with foreign proteins, and often produce symptoms with which we are familiar as "allergies." In essence, the sufferer from multiple sclerosis may be allergic to himself and multiple sclerosis may be an example of an auto-allergic disease.

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Since those nerves that receive sensations are most likely to be attacked, double vision, loss of the ability to feel, and other abnormal sensations are common symptoms. Multiple sclerosis most often attacks people in the 00-10-40 age group. It may be progressive — that is, more and more nerves may become involved, so that death eventually follows. The progress of the disease can be slow, however, death sometimes not following for ten or more years after its onset.

ACETYLCHOLINE

Any neuron does not exist in isolation. It usually makes contact with another neuron. This happens through an intermingling of the axon of one neuron (the axon branches at its extreme tip) with at least some of the dendrites of another. At no point do the processes of one cell actually join the processes of another. Instead there is a microscopic, but definite, gap between the ends of the processes of two neighboring neurons. The gap is called a synapse (sih-naps'; "union" G, although a union in the true sense is exactly what it is not).

This raises a problem. The nerve impulse does indeed travel from one neuron to the next, but how does it travel across the synaptic gap? One thought is that it "sparks" across, just as an electric current leaps across an ordinarily insulating gap of air from one conducting medium to another when the electric potential is high enough. However, the electric potentials involved in nerve impulses {with the exception of certain reported instances in the crayfish) are not high enough to force currents across the insulating gap. Some other solution must be sought, and if electricity will not help we must turn to chemistry.

A very early effect of a stimulus on a nerve is that of bringing about a reaction between acetic acid and choline, two compounds commonly present in cells. The substance formed is aeetykholine (as'ih-til-koh'leen). It is this acetylcholine which alters the working of the sodium pump so that depolarization takes place and the nerve impulse is initiated.

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It is easy to visualize the acetylcholine as coating the membrane and altering its properties. This is the picture some people draw of hormone action in general, and for this reason acetylcholine is sometimes considered an example of a neurohormone — that is, a hormone acting on the nerves. The resemblance, however, is not perfect. Acetylcholine is not secreted into and transported by the bloodstream, as is true in the case of all hormones I described in the first part of this book. Instead, acetylcholine is secreted at the nerve-cell membrane and acts upon the spot. This difference has caused some people to prefer to speak of acetylcholine as a neurohumor, "humor" being an old-fashioned medical term for a biological fluid (from a Latin word for "moisture").

The acetylcholine formed by the nerve cannot be allowed to remain in being for long, because there would be no repolarization while it is present. Fortunately, nerves contain an enzyme called cholinesterase (koh'lin-es'tur-ays) which brings about the breakup of acetylcholine to acetic acid and choline once more. Following that breakup the cell membrane is altered again and repolarization can take place. Both formation and breakup of acetylcholine is brought about with exceeding rapidity, and the chemical changes keep up quite handily with the measured rates of depolarization and repolarization taking place along the course of a nerve fiber.

The evidence for the acetylcholine/cholinesterase accompaniment to the nerve impulse is largely indirect, but it is convincing. All nerve cells contain the enzymes that form acetylcholine and break it down. This means that the substance is found in all multicellular animals except the very simplest: the sponges and jellyfish. In particular, cholinesterase is found in rich concentration in the electric organs of the electric eel, and the electric potential generated by the eel is at all times proportional to the concentration of enzyme present. Furthermore, any substance that blocks the action of cholinesterase puts a stop to the nerve impulse.

\

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And so the picture arises of a nerve impulse that is a coordinated chemical and electrical effect, the two traveling together down the length of an axon. This is a more useful view than that of the nerve impulse as electrical in nature only, for when we arrive at the synapse and find that the electrical effect cannot cross we are no longer helpless: the chemical effect can cross easily. The acetylcholine liberated at the axon endings of one nerve will affect the dendrites, or even the cell body itself, across the synapse and initiate a new nerve impulse there. The electrical effect and chemical effect will then travel down the second neuron together until the chemical effect takes over alone at the next synapse, and so on. (The impulse moves from axon to dendrites and not in the other direction. It is this which forces nerve messages to travel along a one-way route despite the ability of nerve processes to carry the impulse either way.)

The axon of a neuron may make a junction not only with another neuron but also with some organ to which it carries its impulse, usually a muscle. The tip of the axon makes intimate contact with the sarcolemma (sahr'koh-Iem'ub; "skin of the flesh" G), which is the membrane enclosing the muscle fiber. There, in the intimate neighborhood of the muscle, it divides into numerous branches, each ending in a separate muscle fiber. Again there is no direct fusion between the axon and the muscle fibers. Instead, a distinct — if microscopic — gap exists. This synapse-like connection between nerve and muscle is called the neuromuscular junction (or myoneural junction).

At the neuromuscular junction, the chemical and electrical aspects of the nerve impulse arrive. Once more the electrical effect stops, but the chemical effect bridges the gap. The secretion of acetylcholine alters the properties of the muscle cell membrane, brings about the influx of sodium ion, and, in short, initiates a wave of depolarization just like that which takes place in a nerve cell. The muscle fibers, fed this wave by the nerve endings, respond by contracting. All the muscle fibers to which the various branches of a single nerve are connected contract as a

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unit, and such a group of fibers is referred to, consequently, as a motor unit.

Any substance that will inhibit the action of cholinesterase and put an end to the cycle of acetylcholine buildup and breakdown thus will not only put an end to the nerve impulse but will also put an end to the stimulation and contraction of muscles. This will mean paralysis of the voluntary muscles of the limbs and chest and of the heart muscle as well. Death will consequently follow such inhibition quickly, in from two to ten minutes.

During the 1940*5 German chemists, in the course of research with insecticides, developed a number of substances that turned out to be powerful cholinesterase inhibitors. These are deadly indeed. As liquids they penetrate the skin without damage or sensation, and, once they reach the bloodstream, kill quickly. They are much more subtle and deadly than were the comparatively crude poison gases of World War I. Germany did not make use of them in World War II, but under the name "nerve gases" mention is sometimes made of their possible use in World War III (assuming anything is left to be done after the first nuclear stroke and counterstroke).

Nature need not in this case take a back seat to human ingenuity. There are certain alkaloids that are excellent cholinesterase inhibitors and, therefore, excellent killers. There is curare (kyoo-rah'ree), which was used as an arrow poison by South American Indians. (When news of it penetrated the outside world, this gave rise to legends of "mysterious untraceable South American toxins" that filled a generation of mystery thrillers.) Another example of natural cholinesterase inhibitors is the toxins of certain toadstools, including one that is very appropriately entitled "the angel of death."

Even nerve gases have their good side, nevertheless. It sometimes happens that a person possesses neuromuscular junctions across which the nerve impulse travels with difficulty. This condition is myasthenia grouts (my-as-thee'nee-uh gra'vis; "serious

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muscle weakness" G) and is marked by a progressive weakening of muscles, particularly of the face. Here the most likely fault is that the acetylcholine formation at the neuromuscular junction is insufficient, or perhaps that it is formed in normal amounts but is too quickly broken down by cholinesterase. The therapeutic use of a cholinesterase-inhibitor conserves the acetylcholine and can, at least temporarily, improve muscle action.

Although muscle tissue can be stimulated directly and made to contract — by means of an electric current, for illustration — muscle is, under normal conditions, stimulated only by impulses arriving along the nerve fibers. For that reason, any damage to the fibers, either through mechanical injury or as a result of a disease such as poliomyelitis, can result in paralysis. An axon which has degenerated through injury or disease can sometimes be regenerated, provided its neurilemma has remained intact. Where the neurilemma has been destroyed or where the axon is one lacking a neurilemma (as many are) regeneration is impossible. Further, if the cell body of any nerve is destroyed, it cannot be replaced. (Nevertheless, all is not necessarily lost. In 1963 human nerves were transplanted successfully from one person to another for the first time. There is a reasonable possibility that the time will come when "nerve banks" will exist and when paralysis through loss of nerve function can be successfully treated.)

A particular nerve fiber shows no gradations in its impulse. That is, one does not find that a weak stimulus sets up a weak impulse, while a stronger stimulus sets up a stronger impulse. The neuron is constructed to react completely or not at all. An impulse too weak to initiate the nerve impulse does nothing effective; it is "subthreshold." To be sure, some minor changes in the membrane potential may be noted and there may be the equivalent of a momentary flow of current, but that dies out quickly. (If, however, a second subthreshold stimulus follows before that momentary flow dies out, the two together may be strong enough to initiate the impulse.)

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It would seem that a small current of electricity won't last long in the nerve — its resistance is too high. On the other hand, a stimulus just strong enough to initiate the impulse ("threshold stimulus") results in an electrical and chemical effect which is regenerated all along the nerve fiber and does not fade out. (How the regeneration takes place is uncertain, though there is a strong suspicion that the nodes of Ranvier are involved.) The threshold stimulus produces the fiber's maximum response. A stronger stimulus can produce no stronger impulse. This is the "all-or-none law" and may be simply stated: a nerve fiber either propagates an impulse of maximum strength or propagates no impulse at all.

The all-or-none law extends to the organ stimulated by the nerve. A muscle fiber receiving a stimulus from a nerve fiber responds with a contraction of constant amount. This seems to go against common knowledge. If a nerve fiber always conducts the same impulse, if it conducts any at all, and if a muscle fiber always contracts with the same force, if it contracts at all, then how is it we can contract our biceps to any desired degree from the barest twitch to a full and forceful contraction?

The answer is that we cannot consider nerve and muscle isolated either in space or time. An organ is not necessarily fed by a single nerve fiber, but may be fed by dozens of them. Each nerve fiber has its own threshold level, depending on its diameter, for one thing. The larger fibers tend to have a lower threshold for stimulation. A weak stimulus, then, may be enough to set off some of the fibers and not the others. (A stimulus so weak that it suffices to set off only one nerve fiber is the minimal stimulus.} A muscle would merely twitch if a single motor unit would contract under the minimal stimulus. As the stimulus rises, more and more nerve fibers would fire; more and more motor units would contract. Eventually, when the stimulus was strong enough to set off all the nerve fibers (maximal stimulus), the muscle would contract completely. A stimulus stronger than this will do no more.

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There is also the matter of time. If a nerve fiber carries an impulse, the motor unit to which it is connected contracts and then relaxes. The relaxation takes time. If a second impulse follows before relaxation is complete, the muscle contracts again, but from a headstart, so that it contracts further the second time. A third impulse adds a further total contraction, and so on. The faster one impulse follows another the greater the contraction of the muscle. The number of impulses per second that can be delivered by a nerve fiber is very high and depends upon the length of the refractory period. Small fibers have refractory periods of as long as 1/250 of a second, which, even so, means that as many as 250 impulses per second can be delivered. Ten times as many can be delivered by the large myelinated nerve fibers.

In actual fact, a muscle is usually stimulated by some portion of the nerve fibers that feed into it, each fiber firing a certain number of times a second. The result of these two variable effects is that, without violating the all-or-none law, a muscle can be made to contract in extremely fine gradations of intensity.

7

OUR  NERVOUS  SYSTEM

CEPHALIZATION

For nerve cells to perform their function of organizing and coordinating the activity of the many organs that make up a multicellular creature, they must themselves be organized into a nervous system. It is the quality and complexity of this nervous system that more than anything else dictates the quality and complexity of the organism. Man considers himself to be at the peak of the evolutionary ladder, and although self-judgment is always suspect there is at least one good objective argument in favor of this. The nervous system of man is more complex, for his size, than is that of any other creature in existence (with the possible exception of some cetaceans). Since our nervous system is the clearest mark of our superiority as a species, I think it is important to see how it came to develop to its present state.

The simplest creatures that possess specialized nerve cells are the coelenterates (sih-len'tur-ayts), which include such organisms as the freshwater hydra and the jellyfish. Here already there is a nervous system of sorts. The neurons are scattered more or less regularly over the surface of the body, each being connected by synapses to those nearest to it. In this fashion, a stimulus applied to any part of the creature is conducted to all parts. Such a nervous system in a sense is merely an elaboration on a larger scale of what already existed in unicellular creatures. Among them, the

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cell membrane is itself excitable and conducts the equivalent of a nerve impulse to all parts of itself. The nerve network of the coelenterate does the same thing, acting as a supermembrane of a supercell. The results of such an arrangement, however, represent no great advance. Any stimulus anywhere on the coelenterate body alerts the entire organism indiscriminately and results in a response of the whole, which proceeds to contract, sway, or undulate. Fine control is not to be expected. Furthermore, since there are so many synapses to be passed (each a bottleneck), conduction of the nerve impulse is in general slow.

The next more complicated group of animals are the flatworms, which, although still simple, show certain developments that foreshadow the structure of all other, more complicated, animals. They are the first to have the equivalent of muscle tissue and to make effective use of muscles; the efficiency of the neuron network hence must be improved. Such improvements do indeed take place in at least some flatworms. The nerve cells in these creatures are concentrated in a pair of nerve cords running the length of the body. At periodic intervals along the length of the cords, there emerge nerves that receive stimuli from or deliver impulses to various specific body regions. The nerve cords represent the first beginnings of what is called the central nervous system, and the nerves make up the peripheral nervous system. This division of the nervous system into two chief portions holds true for all animals more advanced than the flatworm, up to and including man.

In any creature with a central nervous system, a stimulus will no longer induce a response from the whole body generally. Instead, a stimulus at some given part of the body sets off a nerve impulse that is not distributed to all other neurons but is carried directly to the nerve cord. It passes quickly along the nerve cord to a particular nerve, which will activate a specific organ, or organs, and bring about a response appropriate to the original stimulus.

The coelenterate system would correspond to a telephone net-

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work in which all subscribers are on a single party line, so that any call from one to another rouses every one of the subscribers, who are then free to listen and probably do. The flatworm system resembles a telephone network in which an operator connects the caller directly with the desired party. We can see at once that the operator-run telephone network is more efficient than the one-big-party line.

Early in the evolutionary process, however, the nerve cords became more than simple cords. The nerve cord had to specialize, even in the flatworms, and this specialization arose, in all probability because of the shape of the flatwonn. The flatworm is the simplest living multicellular animal to have bilateral symmetry, and its primeval ancestors must have been the first to develop this. (By bilateral symmetry, we mean that if a plane is imagined drawn through the body it can be drawn in such a way as to divide the creature into a right and left half, each of which is the mirror-image of the other.) All animals more complicated than the flatworms are bilaterally symmetrical; ourselves most definitely included. One seeming exception is the starfish and their relatives, for these possess radial symmetry. (In radial symmetry, similar organs or structures project outward from a center, like so many radiuses in a circle.) The seeming exception is only a seeming one, for the radial symmetry of the starfish is a secondary development in the adult. The young forms (larvae) exhibit bilateral symmetry; the radial symmetry develops later as a kind of regression to an older day.

Animals simpler than the flatworms, such as the coelenterates, sponges, and single-celled creatures, generally have either radial symmetry or no marked symmetry at all. The same is true of plants: the daisy's petals are a perfect example of radial symmetry and the branches of a tree extend unsymmetrically in all directions from the trunk. This grand division of living creatures into those with bilateral symmetry and those without is of vital importance. An animal with no marked symmetry or with radial symmetry need have no preferred direction of movement. There is no reason

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why one particular leg of a starfish should take the lead in movement over any other.

A creature with bilateral symmetry is usually longest in the direction of the plane of symmetry and tends to progress along that plane. Other directions of movement are possible, but one direction is preferred. If a bilaterally symmetric creature, by reason of its very shape and structure, adopts one preferred direction of movement, then one end of its body is generally breaking new ground as it moves. It is constantly entering a new portion of the environment. It is that end of the body which is the head.

Obviously, it is important that the organism have ways of testing the environment in order that appropriate responses be made, responses of a nature to protect its existence. It must be able to check the chemical nature of the environment, avoiding poison and approaching food. It must detect temperature changes, vibrations, certain types of radiation, and the like. Organs designed to receive such sensations are most reasonably located in the head, since that is the ground-breaking portion of the body. It meets the new section of the environment first. The mouth also is most reasonably located in the head, since the head is the first portion of the body to reach the food. The end opposite the head (that is, the tail) is comparatively featureless.

In consequence, the two ends of the bilaterally symmetric creature are in general different, and living creatures of this type have distinct heads and tails. The differentiation of a head region marked by sense organs and a mouth is referred to as cephalization ("head" G), The process of cephalization has its internal effects on the nervous system. If a bilaterally symmetric creature were equal-ended, the nerve cords would, understandably enough, be expected to be equal-ended as well. But with a distinct head region containing specialized sense organs, it would be reasonable to suppose that the nerve cords in that head region would be rather more complex than elsewhere. The nerve endings in the specialized sense organs would be more numerous than elsewhere in the body, and the receiving cell bodies (most logically placed at the

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head end of the cord, since this is the portion nearest the sense organs) would likewise have to be more numerous.

Even in flatworms there is an enlargement and enrichment of the nerve cord at the head end, therefore. Such an enlargement might be called the first and most primitive brain. * Not surprisingly, the brain grows more complex as the organism itself grows more complex. It reaches the pinnacle of its development in the Phylum80 Chordata, the one to which we ourselves belong.

The special position of Chordata with respect to the nervous system is shown in the very nature of the nerve cord. The double nerve cord of the flatworms persists in most phyla. It remains a solid tube in structure and is ventrally located; that is, it runs along the abdominal surface of the body. Only in Chordata is this general scheme radically altered. In place of a solid double nerve cord, there is a single cord in the form of a hollow cylinder. Instead of being ventrally located, it is dorsally located — it lies along the back surface of the body. This single, hollow, dorsal nerve cord is possessed by all chordates (members of the Phylum Chordata), and by chordates only; and, if we judge by results, it is much preferable to the older form that had been first elaborated by the distant ancestors of the flatworms.

THE   CHORDATES

The Phylum Chordata is divided into four subphyla, of which three are represented today by primitive creatures that are not very successful in the scheme of life. In these three, the nerve cord receives no special protection any more than it does in the phyla other than Chordata.

* The word "brain" is from the Anglo-Saxon but it may be related to a Greek word referring to the top of the head. A less common synonym for brain is encephalon (en-sef'uh-lon; "in the head" G). This name is most familiar to the general public in connection with a disease characterized by the inflammation of brain tissue, since this disease is known as encephalitis.

" The animal kingdom is divided into phyla (f/Iuh; "tribe" G, singular, phylum}, each representing a group with one general type of body plan. A discussion of the various phyla and of the development of Chordata is to be found in Chapter i of The Human Body.

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In the fourth and most advanced subphylum of Chordata, on the contrary, the nerve cord receives special protection in the form of enclosure by a series of hard structures of either cartilage or bone. These structures are the vertebrae, and for this reason the subphylum is called Vertebrata (vur'tih-bray'tuh) and its members commonly referred to as the vertebrates.

And it is only among the vertebrate subphylum that the brain becomes prominent. Of the other three snbphyla, the most advanced (or, at least, the one that most resembles the vertebrates) is the one that contains a small fishlike creature called the amphi-oxus. The similarity to fish (resting chiefly on the fact that it has a cigar-shaped body) vanishes upon closer inspection. For one thing, the amphioxus turns out to have not much of a head. One end of it has a fringed suckerlike mouth and the other a finny fringe, and that is about all the difference. The two ends come to roughly similar points. In fact, the very name amphioxus is from Greek words meaning "both-pointed," with reference to the two ends. This lack of advanced cephalization is reflected internally; the nerve cord runs forward into the head region with scarcely any sign of specialization. The amphioxus is virtually a brainless creature.

Among the vertebrates, however the situation changes. Even in the most primitive living class of vertebrates (a class containing such creatures as the lamprey, an organism that has not yet developed the jaws and limbs characteristic of all the other and more complex vertebrate classes) the forward end of the nerve cord has already swelled into a clear brain. Nor is it a simple swelling. It is, rather, a series of three — a kind of triple brain — the swellings being named (from the front end backward) the forebrain, midbrain, and hindbrain. These three basic divisions remain in all higher vertebrates, although they have been much modified and have been overlaid with added structures.

When the vertebrates first developed, some half-billion years ago, the early primitive specimens developed armor covering their head and foreparts generally. Such armor has the disad-

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vantage of adding deadweight to the creature and cutting down speed and maneuverability; among vertebrates generally, tbe development of armor has never been a pathway to success," And yet the brain had to be protected. A compromise was struck whereby the armor was drawn in beneath the skin and confined to the brain alone. In this way the skull was developed.

The vertebrates rely not on the passive defense of a shell but on speed, maneuverability, and the weapons of attack. The central nervous system — the brain and nerve cord — was ex-cepted; for in all vertebrates it is carefully enclosed in cartilage or bone, a shelled organ within an unshelled organism. Certainly this seems a rather clear indication of the special importance of the brain and nerve cord in vertebrates.

The three sections of the brain show further specializations, even in primitive vertebrates. From the lower portion of the most forward portion of the forebrain are a pair of outgrowths which received the nerves from the nostrils and are therefore concerned with the sense of smell. These outgrowths are the olfactory lobes ("smell" L); see illustration, page 196. Behind the olfactory lobes are a pair of swellings on the upper portion of the forebrain, and these make up the cerebrum (sehr'uh-brum; "brain" L). The portion of the forebrain that lies behind the cerebrum is the thalamus (thaVuh-mus)."" The midbrain bears swellings that are particularly concerned with the sense of sight and are therefore termed the optic lobes ("sight" G).

The hindbrain develops a swelling in the upper portion of the region adjoining the midbrain. This swelling is the cerebellum (sehr'uh-bel'um; "little brain" L). The region behind the cerebellum narrows smoothly to the point where it joins the long section of unspecialized nerve cord behind the head. This final region is the medulla oblongata (meh-dul'uh ob'long-gay'tuh;

* Examples of modem armored vertebrates are the turtles, armadillos, and pangolins, all relatively unsuccessful.

** This word is from the Greek and refers to a type of room. The name arose among the Romans who felt this section of the brain was hollow and therefore resembled a room.

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"rather long marrow" L). It is "marrow" in the sense that it is a soft organ set within hard bone, and, unlike other portions of the human brain, is elongated rather than bulgy.

This is the essence of the brain throughout all the classes of Vertebrata. There are shifts in emphasis, though, depending on whether smell or sight is the more important sense. In fish and amphibians smell is the chief sense, so the olfactory lobes are well developed. In birds, smell is comparatively unimportant and sight is the chief sense. In the bird brain, therefore, the olfactory lobes are small and unimpressive, whereas the optic lobes are large and well developed.

The development of the brain into something more than a sight-and-smell machine involved the cerebrum primarily. The outer coating of the cerebrum, containing numerous cell bodies that lend the surface a grayish appearance, is the cerebral cortex ("cortex" being the Latin word for outer rind, or bark), or the palUum ("cloak" L). It can also be termed, more colloquially, the "gray matter." This, in fish or amphibians, is chiefly concerned with sorting out the smell sensations and directing responses that would increase the creature's chance to obtain food or escape an enemy.

In the reptiles the cerebrum usually is distinctly larger and more specialized than in fish or amphibians. One explanation for this may well be that the dry-land habitat of the reptile is much more hostile to life than the ocean and fresh water in which the older classes of Vertebrata developed. On land, the medium of air is so much less viscous than that of water that faster movement is possible, which in itself requires more rapid coordination of muscular action. In addition, the full force of gravity, unneu-tralized by buoyancy, presents greater dangers and again places a premium on efficient muscle action.

Therefore, although the reptilian cerebrum is still mainly concerned with the analysis of smell and taste sensations, it is larger, and in the part of the cerebral cortex nearest the front end there is the development of something new. This new portion of the

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cortex is the neopallium (nee'oh-pal'ee-um; "new cloak" L). It consists of tracts of nerve cell bodies involved with the receipt of sensations other than smell. In the neopallium a greater variety of information is received and more complicated coordinations can be set up. The reptile can now move surefootedly, despite the upsetting pull of gravity. The neopallium was developed further in that group of reptiles which, about 100 million years ago or so, underwent some remarkable changes — changing scales into hair, developing warm-bloodedness, and, in general, becoming mammals, the most complex and successful class of Vertebrata.

In primitive mammals the cerebrum is even larger than in reptiles, although remaining just as specialized for reception of smell sensations. At least the pallium remains so. However, there is a large expansion in the size of the neopallium, which spreads out to cover the top half of the cerebral cortex.

The larger the neopallium, which is the center of a great variety of coordinations among stimuli and responses, the more complex the potentialities of behavior. A simple brain may have room for only one response to a particular stimulus; a more complex brain will have room to set up neuron combinations that can distinguish different gradations of a stimulus and take into account the different circumstances surrounding the stimulus, so that a variety of responses, each appropriate to the particular case, becomes possible. It is the presence of an increased variety of response that we accept as a sign of what we call "intelligence." It is the enlarged neopallium, then, which makes mammals in general more intelligent than any other group of vertebrates and, indeed, more intelligent than any group of invertebrates.

During the course of mammalian evolution, there was a general tendency toward an increase in body size. This usually implies an increase in the size of the brain and, as a result, in the cerebrum and neopallium. With increase in size, it is at least possible, therefore, that an increase in intelligence would follow. This is not necessarily so, since, normally, the larger the animal the more

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complex is the coordination required, even when there is no advance in intelligence. Sensations arrive from larger portions of the environment and are therefore more complicated. The larger, heavier, and more numerous muscles require more careful handling. If an animal increases in size without increasing its brain in proportion, and even more than in proportion, it is likely to become more stupid rather than more intelligent.

An extreme example of this was in the giant reptiles of the Mesozoic Era. Some grew larger than any land mammal ever has, but very little of that increase went into the brain. In fact, one of the most startling things about the monsters is the pin-head brain they carried atop their mountains of flesh. There seems no doubt but that they must have been abysmally stupid creatures. In the worst cases, the creatures lacked enough brain to take care of the minimal requirements of muscular coordination. The stegosaur, to name one, which weighed some ten tons or more (larger than any elephant), had a brain no larger than that of a kitten. It was forced to develop a large collection of nerve cells near the base of the spinal cord which could coordinate the muscles of the rear half of the body, leaving only the front half to the puny brain. This "spinal brain" was actually larger than the brain in the skull. The phenomenon of large size associated with decreased intelligence exists in mammals, too, though in not nearly as marked a fashion. The large cow is a rather stupid mammal, not nearly so bright as the comparatively small dog.

Some mammals, as they increased in size, enlarged the area of the neopallium more than in proportion, so they increased in intelligence as well. To enlarge the neopallium at that rate, however, meant that it would outgrow the skull. With the larger and more recently developed mammals, therefore, the cerebral cortex, which by then had become all neopallium, must wrinkle. In place of the smooth cerebral surface found in all other creatures, even among the smaller and more primitive mammals, the cerebral surface of the larger and higher mammals rather resem-

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bles a large walnut. The surface has folded into convolutions ("roll together" L). The gray matter, following the ins and outs of the wrinkles, increased in area.

In terms of sheer bulk, brain growth reaches an extreme in the very largest mammals, the elephants and whales. These have the largest brains that have ever existed. What is more, the cerebral surface is quite convoluted; the brains of some of the whale family are the most convoluted known. It is not surprising that both elephants and whales are unusually intelligent animals. Yet they are not the most intelligent. Much of their large brain — too much — is the slave of the coordination requirements of their muscles. Less is left for the mysterious functions of reason and abstract thought.

To look for a record intelligence, then, we must find a group of animals that had developed large brains without neutralizing this by developing excessively huge bodies. What we want, iri other words, is a large value for the brain /body mass ratio.

THE   PRIMATES

For such an increase in brain-body mass ratio, we must turn to that Order of mammals called Primates (pry-may'teez), usually referred to, in Anglicized pronunciation, as the primates (pry'mits). The term is from the Latin word for "first," a piece of human self-praise, since included in the Order is man himself.

About 70 million years ago, the primates first developed out of the Order Insectivora (in-sek-tiv'oh-ruh; "insect-eating" L). The living examples of the insectivores are small and rather unremarkable creatures such as the shrews, moles, and hedgehogs, and the earliest primates could not have been much different from these. As a matter of fact, the most primitive living primates are small animals, native to Southeast Asia, called tree-shrews. They are indeed much like shrews in their habits, but are larger (the

15O    THE    HUMAN    BRAIN

shrews themselves are the smallest mammals). They are large enough to remind people of diminutive squirrels, so they are sometimes called "squirrel-shrews" and are placed in the Family Tupaiidae (tyoo-pay'ih-dee; from a Malay word for "squirrel"). Their brains are somewhat more advanced than those of ordinary insectivores and they possess various anatomical characteristics which to a zoologist spell "early primate" rather than "late insec-tivore."

An important difference between the tree-shrew and the shrew lies in just that word "tree." The primates began as arboreal (tree-living) creatures, and all but some of the larger specimens still are. The arboreal environment is the land environment exacerbated still further, to almost prohibitive difficulty. The hard land is at least steady and firm, but the branches of trees do not offer a continuous surface and sway under weight or in the wind. The dangers of gravity are multiplied, too. In case of a misstep, an organism does not merely fall the height of its legs; it falls from the much greater height of the branch.

There are a number of ways in which mammals can adapt to the difficult arboreal life. Reliance can be placed on smallness, nimbleness, and lightness. For a squirrel, the thin branches are negotiable and the danger of a fall is minimized. (The smaller a creature, the less likely it is to be hurt by a fall.) With the development of a gliding surface of skin, as in the "flying" squirrel, the fall is actually converted into a means of locomotion. An alternative is to trade nimbleness for caution, to move very slowly and test each step as it is taken. This is a solution adopted by the sloths, which have attained considerable size at the cost of virtually converting themselves into mammalian turtles.

The early primates took the path of the squirrel. These include not only the tree-shrews but also the lemurs (lee'merz; "ghost" L, because of their quiet and almost ghostly movements through the night — they being nocturnal creatures). Together, the tree-shrews and the lemurs are placed in the Suborder Prosimii (proh-sim'ee-eye; "pre-monkeys" L).

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The whole Suborder is still marked by its insectivorous beginnings. The members have pronounced muzzles, with eyes on either side of the head; the cerebral cortex is still smooth and is still mainly concerned with smell. Notwithstanding, a crucial change was taking place. Slowly, more and more, the early primates tackled the difficulties of arboreal life head on. They did not follow a course of evasions. They did not merely patter along the branch but developed a grasping paw — that is, a hand — with which to seize the branch firmly.

Nor did they escape the dangers of gravity by developing a gliding membrane.0 Rather, they relied on an improvement of the coordination of eye and muscle. In judging the exact position of a swaying branch, no sense is as convenient as that of sight, and even among the tree-shrews a larger proportion of the brain is devoted to sight and a lesser amount to smell than is true of the insectivores. This tendency continues among the lemurs.

The most specialized of the lemurs is the tarsier (tahr'see-ur; so called because the bones of its tarsus, or ankle, are much elongated ). Here the importance of sight as opposed to sound makes itself evident in a new way. The eyes are located in front rather than on either side of its face. Both can be brought to bear simultaneously on the same object and this makes stereoscopic, or three-dimensional, vision possible. Only under such conditions can the distance of a swaying branch be estimated with real efficiency. (The tarsier's eyes are so large for its tiny face that its silent, staring appearance at night has given it the name "spectral tarsier.") With the development of grasping hands capable of holding food and bringing it to the mouth, the importance of the muzzle declines; the tarsier in reality lacks one, and is flat-faced like a man. The decline of the muzzle, together with the ascendancy of the eye, allows the sense of smell to become less important.

" There is an animal called a flying lemur, which has developed such a membrane, but it is an insectivore and not a primate.

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The suborder including all the remaining primates is Anthro-poidea (an'throh-poi'dee-uh; "manlike" G), and within this grouping are the monkeys, apes, and man himself. Among all of these, the traits in the tarsier are accentuated. All are primarily eye-and-hand-centered, with stereoscopic vision and with the sense of smell receding into the background.

Of all the senses, sight delivers information to the brain at the highest rate of speed and in the most complex fashion. The use of a hand with the numerous delicate motions required to grasp, finger, and pluck requires complex muscle coordination beyond that necessary in almost any other situation. For an eye-and-hand animal to be really efficient there must be a sharp rise in brain mass. If such a rise had not come about the primates would have remained a small, inconspicuous, and unsuccessful group with eyes and hands incapable of developing their full potentiality. But there was a sharp rise in brain mass. No other animal the size of a monkey has anywhere near the mass of brain a monkey possesses. (For their body size, the smaller monkeys have larger brains than we do.) Nor has any other animal its size a brain as convoluted as that of a monkey.

The higher primates are divided into two large groups, the Platyrrhina (plafih-r/nuh; "flat-nosed" G) and Catarrhina (kat'uh-ry'nuh; "down-nosed" G). In the former, the noses are flat, almost flush against the face, with the nostrils well separated and opening straight forward. In the latter, the nose is a prominent jutting feature of the face, and within it the two nostrils are brought close together, with the openings facing downward, as in our own case.

The platyrrhines are to be found exclusively on the American continents and are therefore usually referred to as "New World monkeys." They frequently possess a prehensile tail; one able to curl about a branch and bear the weight of the body even without the supporting help of any of the legs. These prehensile-tailed monkeys are a great favorite at the zoos because of their breathtaking agility. Their four limbs are long and are all

OUR    NERVOUS     SYSTEM   153

equipped for grasping.' The tail acts as a fifth limb. Often the tail and all the limbs are long and light, with a small body at the center. One common group of platyrrhines is commonly known as "spider monkeys" because of their slight, leggy build.

This is all very well as far as adaptation to arboreal life is concerned, but long arms that can stretch from branch to branch reduce the importance of the eye. A tail that can act almost literally as a crutch does the same. The adaptation to arboreal life is wonderful, but in effect some of the pressure is off the brain. The platyrrhines are the least intelligent of the higher primates.

The catarrhines are confined to the Eastern Hemisphere and are therefore ordinarily termed the "Old World monkeys." No catarrhine has a prehensile tail, which means one crutch less. The catarrhines are stockier in build and lack some of the advantageous agility of the platyrrhines. They are forced to make up for it by greater intelligence. The catarrhines are divided into three families. Of these the first is Cercopithecidae (sir'koh-pih-thee'sih-dee; "tailed-monkey" G). This family, as the name implies, possesses tails, though not prehensile ones. The most formidable of the family are the various baboons, which are stocky enough to have abandoned the trees for the ground but have not abandoned the tree-developed eye-and-hand organization, or the intelligence that developed along with it. In addition to their intelligence, they travel in packs and have redeveloped muzzles that are well equipped with teeth.

And yet even the baboons with their short tails must take a backseat in intelligence with respect to the remaining two families of the catarrhines. In the remaining two families there are no tails at all, and the hind legs become increasingly specialized for support rather than for grasping. It is as though intelligence

* Indeed, an old-fashioned name for monkeys is Qvadmmana (kwod-roo'muh-nuh; "four handed" L), and this came to be applied to all higher primates except man. The term has fallen into disuse because this grand division between man on the one hand and all other higher primates on the other is zoologically unsound, however soothing it may be to our pride.

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OUR    NERVOUS    SYSTEM

155

is forced to increase as the number of grasping appendages sinks from five to four, and then from four to two.

APES AND MEN

The second catarrhine family is Pongidae fpon'jih-dee), which includes the animals known as "apes." The name of the family is from a Congolese word for ape. The apes are the largest of the primates and therefore possess the largest brains in absolute terms. This, too, is a factor in making them the most intelligent of all the "lower animals."

There are four types of creatures among the apes. These are, in order of increasing size, the gibbon, the chimpanzee, the orangutan, and the gorilla. The gibbons (of which there are several species) are, on the average, less than three feet high and weigh between 20 and 30 pounds. Furthermore, they have gone nearly the way of the spider monkey. Although without tails, their forearms have lengthened almost grotesquely and they can make their way through the trees, hand-over-hand, with an uncanny accuracy that makes them a fascination in zoos. Between a small size and an overdependence on long limbs, it is not surprising that the gibbon is the least intelligent of the apes.

The remaining three pongids, approaching or even exceeding man in size, are lumped together as the "great apes." The weight of the brain of an orangutan has been measured at about 340 grams (12 ounces), that of a chimpanzee at 380 grams (13)1 ounces), and that of a gorilla at 540 grams (19 ounces, or just under itf pounds). Of these, the most intelligent appears to be the chimpanzee, since the greater mass of the gorilla's brain seems to be neutralized to an extent by the much greater mass of its body.

The similarities between the apes (particularly the chimpanzee) and man are unmistakable and sufficiently apparent to cause the Pongidae to be referred to frequently as the anthropoid apes ("manlike" G). And yet there are distinctions of considerable im-

portance between apes and man, enough difference so that, quite fairly and without too much self-love on our own part, man may be put into a third catarrhine family all by himself, Hominidae (hoh-min'ih-dee; "man" L). Some millions of years ago the creatures ancestral to man branched off from the line of evolution leading to the modern apes. It was from this branch of the family that the first hominids developed. The hominids stood upright, finally and definitely. The hind legs became fully specialized for standing only and it is solely as a kind of stunt that a modern man, for instance, can pick up anything with his small, hardly maneuverable toes. The hominids became definitely two-handed, and the arms did not, as in the gibbon's case, unduly specialize themselves for a single function. The one specialization, that of the opposable thumb, rather served to accentuate the Jack-of-all-trades ability.

The loss of equipment again placed the accent on the brain. The hominids advanced in size, to be sure, outstripping the gibbon, and equaling or even slightly surpassing the chimpanzee. The hominids never attained the weight of the orangutan or the gorilla, and yet the hominid brain developed almost grotesquely; the expanding cranium came to overshadow the shrinking face.

The skull of the oldest creature we can definitely consider a hominid was discovered in Tanganyika in 1959. It has been given the name Zinjanthropus (zin-jan'throh-pus; "East Africa man"), from the native term for East Africa, the region in which it was discovered. The Zinjanthropus skull is much more primitive than any living human skull but also it is much more advanced than any living ape skull. Associated with the fossil, in the strata in which it was found, were tools. Therefore Zinjanthropus was a tool-making creature and deserves the name "hominid" in the cultural as well as the zoological sense. In 1961 the age of the strata in which Zinjanthropus was found was determined by measuring the radioactive decay of the potassium within it. The fossil, it would seem, is 1,750,000 years old. This is quite startling, because until the moment of the time measurement it had

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been felt that tool-making hominids had been inhabiting the planet only for half a million years or so. However, the finding is somewhat controversial, and perhaps the last word has not been said,

Zinjanthropus is an example of a small-brained hominid and there are other varieties, too, such as those popularly known as Java man and Peking man from the sites at which the first skeletal remains were discovered. The reference "small-brained" is only relative. Brought to life, such hominids would indeed look pinheaded to us, but their brains approached the looo-gram (2?* pounds) mark and were nearly twice the size of that of any living ape.

Nevertheless, as the hominid group continued to evolve, the accent continued to be on the brain. What we might call the "large-brained hominid" developed, and it was these only which survived to inherit the earth. Today (and since well before the dawn of history) only one species of hominid remains. This is Homo sapiens (hoh'moh say*pee-enz; "man, the wise" L), to whom we can refer as "modern man."

The specimens of modern man existing today are not actually the largest-brained hominids. An early variety of modern man, called the Cro-Magnons (kroh-man'yon; from the region in France where their skeletal remains were first discovered), seems to have the record in this respect. Even Neanderthal man (nay-ahn'der-tahl; so called from the region in Germany where their skeletal remains were first discovered), who seems distinctly more primitive than modern man in skull formation and in jaws, had a brain that was slightly larger than our own. However, between the Neanderthals and ourselves there seemed to be an improvement not so much in size of the brain as in internal organization. In other words, those portions of the brain believed to be most important for abstract thought seem better developed and larger in modem man than in Neanderthal man.

(Nevertheless, there are some who fear that the human brain may have reached its peak and may be on the point of begin-

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158     THE    HUMAN    BRAIN

ning a downhill slide. They point out that individual intelligence is no longer of key importance, since all members of a society, intelligent or not, benefit from the accomplishments of the intelligent few, while those few are forced to pay the penalty of nonconformity by leading a less comfortable life. Evolutionary pressures would therefore now favor declining intelligence. This, however, may be an overly pessimistic view. I hope so, certainly.)

In modern man the brain at birth is about 350 grams (12 ounces) in weight and is already as large as that of a full-grown orangutan. In maturity, the average weight of a man's brain is !45° grams (3^ pounds). The average weight of a woman's brain is about 10 per cent less on the average, but her body is smaller too and there is no reason for thinking that either sex is inherently the more intelligent. Among normal human beings, in fact, there can be marked differences in weight of brain without any clear correlation in intelligence. The Russian novelist Ivan Turgenev had a brain that was just over the 2ooo-gram mark in weight, but Anatole France, also a skilled writer, had one that was just under the i2oo-gram mark.

This represents the extremes. Any brain that weighs as little as 1000 grams is apparently below the minimum weight consistent with normal intelligence and is sure to be that of a mental defective. On the other hand, there have been mental defectives with brains of normal or more than normal size. Weight of brain alone,-although a guide to intelligence, is by no means the entire answer.

If we consider the average weight of a man as 150 pounds and the average weight of his brain as 3)* pounds, then the brain-body ratio is about 1:50. Each pound of brain is in charge (so to speak) of 50 pounds of body. This is a most unusual situation. Compare it with the apes, for example — man's nearest competitors. A pound of chimpanzee brain is in charge of 150 pounds of chimpanzee body (differently expressed, the brain/body ratio is 1:150), whereas in the gorilla the ratio may be as low as 1:500. To be sure, some of the smaller monkeys and also some of the

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hummingbirds have a larger brain/body ratio. In some monkeys the ratio is as great as i: iy)i. If such a monkey were as large as a man and its brain increased in proportion, that brain would weigh 8)s pounds. The brain of such a monkey is in actual fact very small; so small that it simply lacks the necessary mass of cortical material to represent much intelligence despite the small amount of body it must coordinate.

Two types of creatures have brains considerably larger in terms of absolute mass than the brain of man. The largest elephants can have brains as massive as 6000 grams (about 13 pounds) and the largest whales can have brains that reach a mark of 9000 grams (or nearly 19 pounds). The size of the bodies such brains have to coordinate is far, far larger still. The biggest elephant brain may be 4 times the size of the human brain, but the weight of its body is perhaps 100 times that of the human body. Where each pound of our own brain must handle 50 pounds of our body, each pound of such an elephant's brain must handle nearly half a ton of its body. The largest whales are even worse off: each pound of their brain must handle some five tons of body.

Man strikes a happy medium, then. Any creature with a brain much larger than man's has a body so huge that intelligence comparable to ours is impossible. Contrarily, any creature with a brain 'body ratio much larger than ours has a brain so small in absolute size that intelligence comparable to ours is impossible.

In intelligence, we stand alone! Or almost. There is one possible exception to all this. In considering the intelligence of whales, it is perhaps not fair to deal with the largest specimens. One might as well try to gauge the intelligence of primates by considering the largest member, the gorilla, and ignoring a smaller primate, man. What of the dolphins and porpoises, which are pygmy relatives of the gigantic whales? Some of these are no larger than man and yet have brains that are larger than man's (with weights up to 1700 grams, or 3?J pounds), and more extensively convoluted.

It is not safe to say from this alone that the dolphin is more

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intelligent than man, because there is the question of the internal organization of the brain. The dolphin's brain (like that of Neanderthal man) may be oriented more in the direction of what we might consider "lower functions."

The only safe way to tell is to attempt to gauge the intelligence of the dolphin by actual experiment. Some investigators, notably John C. Lilly, seem convinced that dolphin intelligence is indeed comparable to our own, that dolphins and porpoises have a speech pattern as complicated as ours, and that a form of inter-species communication possibly may yet be established. If so, it would surely be one of the most exciting developments in human history. So far, the matter remains controversial, and we must wait and see.

8

OUR  CEREBRUM

THE   CEREBROSPINAL   FLUID

Now that I have discussed the nerve cells (the mode of action of which is identical, as far as we can tell, in all animals) and have briefly taken up the manner in which their organization into a nervous system has grown more intricate through the course of evolution, reaching a climax in ourselves, it is time to take up the human nervous system in particular. The central nervous system (the brain plus the spinal cord) is clearly the best protected part of the body. The vertebrae of the spinal column are essentially a series of bony rings cemented together by cartilage, and through the center of those protecting rings runs the spinal cord. At the upper end of the neck, the spinal cord passes through a large opening in the base of the skull and becomes the brain. The brain is surrounded snugly by the strong, dense bone of the skull.

The protection of the bone alone is rather a harsh one, however. One would not like to entrust the soft tissue of the brain to the immediate embrace of bone. Such an immediate embrace fortunately does not exist, since the brain and spinal cord are surrounded by a series of membranes called the meninges (meh-nin'jeez; "membranes" G). The outermost of these is the dura mater (dyoo'ruh may'ter; "hard mother"" L}, and indeed it is

* The use of the word "mother" dates back to a theory of the medieval Arabs, who felt that out of these membranes all the other body membranes were formed.

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hard. It is a tough fibrous structure that lines the inner surface of the vertebral rings of bone and the inner surface of the skull, smoothing and somewhat cushioning the bare bony outlines. Sheets of dura mater extend into some of the major dividing lines within the central nervous system. One portion extends downward into the deep fissure separating the cerebrum into a right and left half; another extends into the fissure dividing the cerebrum and the cerebellum. On the whole, though, the dura mater is a bone lining.

The innermost of the meninges is the pia mater (py'uh may'ter; "tender mother" L). This is a soft and tender membrane that closely lines the brain and the spinal cord, insinuating itself into all the unevennesses and fissures. It is the direct covering of the central nervous system. Between the dura mater and the pia mater is the arachnoid membrane (a-rak'noid; "cob-weblike" G, so called because of the delicate thinness of its structure). Inflammation of the membranes, usually through bacterial or viral infection, is meningitis. Bacterially caused meningitis, in particular, was a very dangerous disease before the modern age of antibiotics. Even the membranes, taken by themselves, are not sufficient protection for the brain: between tbe arachnoid membrane and the pia mater (the subarachnoid space) is the final touch, the cerebrospinal fluid. One way in which the cere-brospinal fluid protects the brain is by helping to counter the effects of gravity. The brain is a soft tissue; as a matter of fact, its outermost portions are 85 per cent water, making this the most watery of all the solid tissues of the body. It is even more watery than whole blood. Hence it is not to be expected that the brain can be very hard or rigid — it isn't. It is so soft that if it were to rest unsupported on a hard surface the pull of gravity alone would be sufficient to distort it. The cerebrospinal fluid supplies a buoyancy that almost entirely neutralizes gravitational pull within the skull. In a manner of speaking, the brain floats in fluid.

The fluid also counters the effect of inertia.  The bony

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work of the skull protects the brain against the direct impact of a blow (even a light tap would suffice to damage the unprotected tissue of the brain). This protection in itself would scarcely be of much use if the sudden movement of the head, in response to a blow, smashed the brain against the hard internal surface of the skull, or even against the fibrous dura mater. It is of little moment whether it is the enemy club that delivers the blow or your own bone. For that matter, even the mere sudden lifting or turning of the head would be enough to press the brain with dangerous force against the skull in the direction opposite the movement. The cerebrospinal fluid acts as a cushion in all these cases, damping the relative motions of brain and skull. The protection isn't unlimited, of course. A strong enough blow or a strong enough acceleration can be too much for the delicate brain structure, even if the disturbance is not suflicient to produce visible damage. Even if the brain is not directly bruised, sudden twisting of the skull (as is produced in boxing through a hard blow to the side of the chin) may stretch and damage nerves and veins as the brain, through inertia, lags behind the turning head. Unconsciousness can result and even death. This is spoken of as concussion ("shake violently" L).

The cerebrospinal fluid is also to be found in the hollows within the brain and spinal cord, and this brings us to another point. Despite all the specialization and elaboration of the human brain, the central nervous system still retains the general plan of structure of a hollow tube, a plan that was originally laid down in the first primitive chordates. Within the spinal cord this hol-lowness is almost vestigial, taking the form of a tiny central canal, which may vanish in adults. This central canal, like the spinal cord itself, broadens out within the skull. As the spinal cord merges with the brain, the central canal becomes a series of specialized hollows called ventricles.* There are four of these,

* "Ventricle," from a Latin word meaning "little belly," can be used for any hollow within an organ. The most familiar ventricles to the average man are those within the heart.

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numbered from the top of the brain downward. Thus, the central canal opens up at the base of the brain into the lowermost of these, the fourth ventricle. This connects through a narrow aperture with the third ventricle, which is rather long and thin.

Above the third ventricle there is a connection, through another narrow aperture, with the two foremost ventricles, which lie within the cerebrum, one on either side of the fissure that divides the cerebrum into a right and left portion. Because of the fact that they lie on either side of the midline they are referred to as the lateral ventricles. The lateral ventricles are far larger than the third and fourth and have a rather complicated shape. They run the length of the cerebrum in a. kind of outward curve, beginning near each other at the forehead end, and separating increasingly as the back of the skull is approached. A projection of each lateral ventricle extends downward and still outward into the lower portion of the cerebrum.

These hollows — the central canal and the various ventricles — are filled with cerebrospinal fluid. The cerebrospinal fluid is very similar in composition to the blood plasma (the liquid part of the blood), and in reality is little more than filtered blood. In the membranes surrounding the ventricles there are intricate networks of fine blood vessels called chorioid plexuses (koh'ree-oid plek'sus-ez; "membrane networks" L). These blood vessels leak, and are the source of the cerebrospinal fluid. The cells and subcellular objects within the blood, such as the white cells, the red corpuscles, and the platelets, do not pass through, of course; the leak isn't that bad. Nor do most of the protein molecules. Virtually all else in the blood does leak out, nevertheless, and pass into the ventricles.

The cerebrospinal fluid circulates through the various ventricles, and in the fourth ventricle escapes through tiny openings into the subarachnoid space outside the pia mater. Where the subarachnoid space is greater than usual, the fluid collects in cisternae f sis'tur-nee; "reservoirs" L). The largest of these is to be found at the base of the brain, just above the nape of the neck; it is the cisterna magna {"large reservoir" L). The total volume

VENTRICLE

BIGHT          =

lATERAl      ^

VENTRICLE           :5

^    INFERIOR MORN

LOCATION  OF

BRAIN  VENTRICLES

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THE    HUMAN    BRAIN

of cerebrospinal fluid amounts to only a few drops in the newborn child but increases to 100 or 150 milliliters (about 4)* fluid ounces) in the adult.

Since cerebrospinal fluid is continually seeping into the ventricles, it must be allowed to escape somewhere. In the arachnoid membrane are small areas called arachnoid villi (vil'eye; "tufts of hair" L, so named because they have the appearance of tiny tufts). These are richly supplied with blood vessels, and into these the cerebrospinal fluid is absorbed. There is a resultant active circulation of the fluid from the chorioid plexuses, where it leaks out of the blood, through the ventricles, out into the sub-arachnoid space, and through the arachnoid villi, where it is absorbed back into the blood.

It is possible for the circulation of the cerebrospinal fluid to be interfered with. There may be blockage at some point, perhaps through the growth of a brain tumor, which closes off one of the narrow connections between the ventricles. In that case, fluid will continue to be formed and will collect in the pinched-off ventricles, the pressure rising to the point where the brain tissue may be damaged. Inflammation of the brain membranes (meningitis) can also interfere with the reabsorption of the fluid and lead to the same results. At such times, the condition is hydro-cephalus (hy'droh-sef'ah-lus; "water-brain" G), or as it is commonly referred to, in direct translation, "water on the brain." This condition is most dramatic when it takes place in early infancy, before the bones of the skull have joined firmly together. They give with the increased internal pressure so that the skull becomes grotesquely enlarged.

Cerebrospinal fluid can be removed most easily by means of a lumbar puncture; that is, by the introduction of a needle between the fourth and fifth lumbar vertebrae in 'the small of the back. The spinal cord itself does not extend that far down the column, therefore the needle may be inserted without fear of damaging spinal tissue. The collection of nerves with which the canal is filled in that region make easy way for the needle. Fluid may be

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obtained, with greater difficulty, from the cisterna magna, or even from the ventricles themselves if conditions are grave enough to warrant drilling a hole through the skull. From the fluid pressure and from its chemical makeup it is possible to draw useful conclusions as to the existence or nonexistence of a brain tumor or abscess, of meningitis or other infection, and so on.

The cerebrospinal fluid offers more than mechanical protection: it is also part of a rather complex system of chemical protection for the brain. The brain, you see, has a composition quite different in some respects from that of the rest of the body. It contains a high percentage of fatty material, including a number of unique components. Perhaps for this reason brain tissue cannot draw upon the material in the blood as freely as other tissues can. It is far more selective, almost as though it had a finicky taste of its own. As a result, when chemicals are injected into the bloodstream it is often true that these chemicals may be quickly found in all the cells of the body except for those of the nervous system. There is a blood-cerebrospinal barrier that seems to prevent many substances from entering the fluid. There is also a direct blood-brain barrier that prevents them from passing from the blood directly into the brain tissue.

The blood-brain barrier may be the result of an extra layer of small cells surrounding the blood capillaries that feed the brain. These cells make up part of the neuroglia (nyoo-rog'lee-uh; "nerve-glue" G) that surrounds and supports the nerve cells themselves. These neuroglial cells, or simply glia cells, outnumber the nerve cells by 10 to i. There are some 10 billion nerve celk in the cerebrum but 100 billion glia, and these latter make up about half the mass of the brain. A coating of these about the capillaries would serve to deaden the process of diffusion between blood and brain and so erect a selective barrier. (It has usually been assumed that these glia cells serve only a supporting and subsidiary function in the brain. Some recent research, however, would make it appear they are more intimately concerned with some brain functions such as memory.)

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The brain is highly demanding in another fashion. It uses up a great deal of oxygen in the course of its labors; in fact, in the resting body, !« of the oxygen being consumed by the tissues is used up in the brain, although that organ makes up only i ^50 of the mass of the body. The consumption of oxygen involves the oxidation of the simple sugar (glucose) brought to the brain by the bloodstream. The brain is sensitive to any shortage of either oxygen or glucose and will be damaged by that shortage sooner than any other tissue. (It is the brain that fails first in death by asphyxiation; and it is the brain that fails in the baby if its first breath is unduly delayed.) The flow of blood through the brain is therefore carefully controlled by the body and is less subject to fluctuation than is the blood flow through any other organ. What is more, although it is easy to cause the blood vessels in the brain to dilate by use of drugs, it is impossible to make them constrict and thus cut down the blood supply.

The existence of a tumor can destroy the blood-brain barrier in the region of the tumor. This has its fortunate aspect. A drug labeled with a radioactive iodine atom and injected into the bloodstream will pass into the brain only at the site of the tumor and collect there. This makes it possible to locate the tumor by detecting the radioactive region.

THE   CEREBRAL   CORTEX

Since we stand upright, our nervous system, like all the rest of us, is tipped on end. Where in other vertebrates the spinal cord runs horizontally, with the brain at the forward end, in ourselves the cord runs vertically, with the vastly enlarged brain on top. During the course of the development of the nervous system, new — and what we might call "higher" — functions {involving the more complex types of coordination and the ability to reason and indulge in abstract thought) were added to the forward end of the cord through the process of cephalization. Since in the human being the forward end is on top, it follows that when we speak of

CEREBRUM

FROM   BELOW

BACK   VIEW

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higher and lower levels of the central nervous system we mean this both literally and in some ways figuratively.

Furthermore, in the human being the higher levels have come to predominate not only in terms of our own estimate of importance but in that of actual mass. The central nervous system in the average man weighs about 1480 grams, or just over 3 pounds. Of this, the spinal cord — the lowest and most primitive level — weighing about an ounce, makes up only 2 per cent of the total. And of the brain, the cerebrum — which stands at the highest level and is the most recently developed — makes up 5/6 of the total mass.

In describing the central nervous system in detail, then, let us begin with the cerebrum, which is almost divided, longitudinally, into right and left halves, each of which is called a cerebral hemisphere. The outermost layer of the cerebrum consists of cell bodies which, grayish in color, make up most of the gray matter of the brain. This outermost layer of gray matter is the cerebral cortex (where "cortex" has the same meaning it has in the case of the adrenal cortex mentioned on page 41). Below the cortex are the nerve fibers leading from the cell bodies to other parts of the brain and to the spinal cord. There are also fibers leading from one part of the cortex to another. The fatty myelin sheath of these fibers lend the interior of the cerebrum a whitish appearance, and this is the white matter of the brain.

The cortex is intricately wrinkled into convolutions, as I mentioned in the previous chapter. The lines that mark off the convolutions are called sulci (sul'sy; "furrows" L), the singular form being sulcus (sul'kus). Particularly deep sulci are termed fis-sures. The ridges of cerebral tissue between the sulci, which look like softly rolled matter that has been flattened out slightly by the pressure of the skull, are called gyri (j'y'ry; "rolls" L), the singular form being gyrus (jy'rus). The convoluted form of the cerebrum triples the surface area of the gray matter of the brain. There is twice as much gray matter, that is, lining the various sulci and fissures as there is on the fiattish surface of the gyri.

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The sulci and gyri are fairly standardized from brain to brain, and the more prominent ones are named and mapped. Two particularly prominent sulci are the central sulcus and lateral sulcus, which occur, of course, in each of the cerebral hemispheres, (The cerebral hemispheres are mirror images as far as the details of structure are concerned.) The central sulcus begins at the top of the cerebrum, just about in the center, and runs curvingly downward and forward. It is sometimes called the fissure of Rolando, after an 18th-century Italian anatomist, Luigi Rolando, who was the first to describe it carefully. The lateral sulcus starts at the bottom of the hemisphere about one third of the way back from the forward end and runs diagonally upward on a line parallel to the base of the cerebrum. It comes to an end after having traversed a little over half the way to the rear of the cerebrum. It is the most prominent of all the sulci, and is sometimes called the fissure of Sylvius, after the professional pseudonym of a i/th-century French anatomist who first described it. (See the illustration on page 172.)

These two fissures are used as convenient reference points by which to mark off each cerebral hemisphere into regions called lobes. The portion of the cerebral hemisphere lying to the front of the central sulcus and before the point at which the lateral sulcus begins is the frontal lobe. Behind the central sulcus and above the lateral sulcus is the parietal lobe (puh-ry'ih-tal). Below the lateral sulcus is the temporal lobe. In the rear of the brain, behind the point where the lateral sulcus comes to an end, is the occipital lobe (ok-sip'ih-tal). The name of each lobe is that of the bone of the skull which is approximately adjacent to it.

It seems natural to think that different parts of the cortex might control different parts of the body, and that through careful investigation the body might be mapped out on the cortex. One of the early speculators in this direction was an i8th-igth century Viennese physician named Franz Joseph Gall. He believed the brain was specialized even to the extent that different sections controlled different talents or temperamental attri-

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MOTOR    AREA

LOWER   LIMB

TRUNK UPPER *._ LIMB          "~-

FISSURE   OF ROLANOO

FISSURE   Of SYLVIUS

AUDITORY HEARING         AREA

SIGHT

LOCATION  OF  IMPORTANT  AREAS   OF  CEREBRUM

LOCATION  OF IMPORTANT AREAS  OF CEREBRUM

butes. Hence, if a portion of the brain seemed more than normally well developed, the talent or attribute should be correspondingly noticeable. His students and followers went further than he did. They conceived the notion that a well-developed portion of the brain would be marked by a corresponding bump on the skull, the bump being required in order to leave room for the overdevelopment of gray matter in that region. It followed that by taking careful note of the fine detail of the shape of the skull, so went the theory, one could tell a great deal about the owner of that skull. So began the foolish pseudoscience phrenology ("study of the mind" G).

But if Gall, and especially his followers, went off on a wrong turning, there was nevertheless something in their notion. In 1861 a French surgeon named Pierre Paul Broca, by assiduous

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postmortem study of brains, was able to show that patients with an inability to speak or to understand speech — a disorder called aphasia ("no speech" G} — possessed physical damage to a particular area of the brain. The area involved was found to be the third left frontal gyrus, which is often called Broca's convolution as a result.

Following that, in 1870, two Germans, Gustav Fritsch and Eduard Hitzig, began a line of research in which they stimulated different portions of the cerebral cortex of a dog in order to take note of what muscular activity, if any, resulted. (It was also possible to destroy a patch of the cortex and to take note of what paralysis might or might not result.) In consequence, the skeletal muscles of the body were to a certain extent mapped out on the cortex.

It was discovered by such lines of investigation that a band of the cortex lying in the frontal lobe just before the central sulcus was particularly involved in the stimulation of the various skeletal muscles into movement. This band is therefore called the motor area. It seems to bear a generally inverted relationship to the body: the uppermost portions of the motor area, toward the top of the cerebrum, stimulate the lowermost portions of the leg; as one progresses downward in the motor area, the muscles higher in the leg are stimulated, then the muscles of the torso, then those of the arm and hand, and finally of the neck and head.

The cerebral cortex in the motor area, as elsewhere, is composed of a number of layers of cells which are carefully distinguished by anatomists. One of these layers contains, in each hemisphere, some 30,000 unusually large cells. These are called pyramidal cells (from their shape), or Betz cells (after the Russian anatomist Vladimir Betz, who first described them in 1874). The fibers from these cells stimulate muscular contractions, each pyramidal cell controlling a particular portion of a particular muscle. Fibers from the smaller cells in the layers above the pyramidal cell layer do not by themselves stimulate muscle contraction, but instead seem to sensitize the muscle fibers so that

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they will more easily and readily respond to pyramidal stimulation.

The fibers from the motor area form into a bundle called the pyramidal tract, or pyramidal system. This leads downward through the various portions of the brain below the cerebrum and into the spinal cord. Because the tract connects the cortex and the spinal cord it is also called the corticospinal tract. The two portions of the tract, one from each cerebral hemisphere, happen to cross each other in the lower regions of the brain and in the uppermost portion of the spinal cord. The result is that stimulation of the motor area of the left cerebral hemisphere results in an effect in the right side of the body, and vice versa.

The existence of the pyramidal system is an indication of the way in which the nervous system is bound into a functional unit. That is, there may be separate anatomical parts, the cerebrum, the cerebellum, and others (which I shall discuss in some detail) but it is not to be supposed that each has a distinct and separate function. Rather, the pyramidal system in its control of motion draws on all parts of the central nervous system, from cortex to spinal column. There are also nerve fibers involved in the control of motion which do not stem from the pyramidal cells, and these, the extra-pyramidal system, likewise connect with all parts of the central nervous system. While the nervous system may be sliced up anatomically in a horizontal fashion, it is much better to slice it in a vertical fashion, functionally.

At each step of the descent from the motor area of the cortex, down through the lower regions of the pyramidal and extra-pyramidal systems to the muscle fibers themselves, there is a multiplication of effect. The fiber from a single pyramidal cell will exert an effect on a number of cells in the spinal cord. Each of these spinal cells will control a number of neurons in the peripheral nervous system (the portion outside the brain and spinal cord) and each neuron will control a number of muscle fibers. All in all, a single pyramidal cell may end up in indirect charge of possibly 150,000 muscle fibers. This helps in the coordination of muscular activity.

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By controlling the amount of this "divergence," the body can be subjected to varying degrees of "fine control" as required. In this way the motion of the torso can be controlled adequately by relatively few pyramidal cells, since the necessary variety of motion is quite limited. Divergence is great here, and one pyramidal cell controls many thousands of fibers. A special situation exists with the fingers, which must be capable of delicately controlled motions of many varieties. Here there is considerably less divergence, and pyramidal cells are in control of definitely fewer muscle fibers.

But the cortex is not involved in merely controlling responses. To make the responses useful ones, it must also receive sensations. In the parietal lobe, therefore, just behind the central sulcus, is a band which is called the sensory area (see illustration, p. 172, for location of this and areas described below).

Despite this name, it does not receive all sensations. The sensations arising from the nerve endings in the skin and in the interior of the body are led through bundles of fibers up the spinal cord and into the brain. Some are sidetracked by the spinal cord itself, others by the lower portions of the brain. Most, however, finally reach the cortex. Those reaching the sensory area are primarily the sensations of touch and temperature, together with impulses from the muscles which give rise to knowledge concerning body position and equilibrium. These are the generalized body senses not requiring any specialized sensory organs.0 The sensory area is therefore sometimes referred to in more limited fashion as the somesthetic area (soh'mes-thet'ik; "body sensation" G). Even this is overgenerous, since one important somesthetic sense, that of pain, is not represented here; it is received in lower portions of the brain. The fact that the sensations are received at various horizontal levels of the nervous system shows that here, too, there is a longitudinal unification of function. There is a reticular activating system which coordinates the various levels in their task of receiving sensations.

* I shall discuss these senses and the others, too, in some detail in Chapters 10, 11, and 12.

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As in the case of the motor area, the regions of the sensory area in the cerebral cortex are divided into sections that seem to bear an inverse relation to the body. Sensations from the foot are at the top of the area, followed successively as we go downward with sensations from the leg, hip, trunk, neck, arm, hand, fingers. Below the area that receives finger sensations are the areas receiving sensations from the head. Lowest of all are the sensations from the tongue; here one specialized sense is involved, because it is in the lowest portion of the sensory area that taste is received. (The other chemical sense, that of smell, is received in a region at the floor of the frontal lobe — the remnant in man of the extensive olfactory lobes in most other vertebrates.)

The sections of the sensory area devoted to the lips, tongue, and hand are (as one might expect) larger in proportion to the actual size of those organs than are the sections devoted to other parts of the body. In fact, distorted little men are sometimes drawn along diagrams of sections of the brain in an attempt to match up graphically the cortex and the body. Both in the motor area and the sensory area, the result is a tiny torso to which a small leg with a large foot is attached in the direction leading to the top of the cranium, and a large arm with a still larger hand is attached in the other direction. Beneath is a large head that seems all mouth and tongue.

This is reasonable enough. As far as movements are concerned, the manipulations of the mouth and tongue that make speech possible and the manipulation of the hand that makes tool-wielding possible are what have been the main factors in making man. As for the senses, one must expect that the flexible manipulation of a hand could not be fully efficient if we did not know, at every moment and in great detail, what it was feeling. The senses related to the mouth are less distinctively human, but while food remains important {and it does, even to intellectual Homo sapiens) sensations from the mouth area will require great attention.

Each of the two important specialized senses, sight and hear-

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ing, has a separate lobe reserved to itself. That portion of the temporal lobe just beneath the sensory area is reserved for the reception of sound and is therefore the auditory area ("hear" L), or the acoustic area ("hear" G). The occipital lobe carries the visual area, which receives and interprets the sensation of sight. This is located at the extreme rear of each cerebral hemisphere.

ELECTROENCEPHALOGRAPHY

There are some 10 billion nerve cells in the cerebral cortex, as I have said, and all are capable of undergoing the chemical and electrical changes that accompany the nerve impulse. (If they didn't, they would be dead.) A specific cell would conduct an impulse only when stimulated, and it is only then, at perhaps rare intervals, when it would be undergoing variations in electrical potential. However, at any given moment a sizable fraction of the 10 billion cells would be firing, so the brain as a whole is constantly active.

Under ordinary conditions, sensations are constantly being funneled into the cerebrum and motor impulses are constantly being sprayed outward. Even if many sensations were cut off, if you were surrounded by complete darkness and silence, if there were nothing to smell or taste, if you were floating weightless in space and could feel nothing, there would still be sensations arising from your own muscles and joints to tell you the relative position of your limbs and torso. And even if you were lying in complete relaxation, moving no muscle consciously, your heart must still pump, the muscles of your chest still keep you breathing, and so on.

It is not surprising that at all times, awake or asleep, the brain of any living creature, and not of man only, must be the source of varying electric potentials. These were first detected in 1875 by an English physiologist, Richard Caton. He applied electrodes directly to the living brain of a dog on which he had operated for the purpose and could just barely detect the tiny currents.

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During the half century after his time, the techniques of detecting and amplifying tiny changes in electric potential improved vastly. By the 1920*5 it was possible to detect the currents even through the thickness of skin and bone covering the brain.

In 1924 an Austrian psychiatrist named Hans Berger placed electrodes against the human scalp and found that by using a very delicate galvanometer he could just detect electric potentials. He waited until 1929 before publishing his work. Since then, the use of more sophisticated technology has made the measurements of these currents a routine affair. The process is called electroencephalography (ee-lek'troh-en-sef'uh-log'ruh-fee; "electric brain-writing" G). The instrument used is an electro-encephalograph, and the recording of the fluctuating potentials is an electroencephalogram. The abbreviation EEG is commonly used to represent all three words.

The electric potential of the brain waves {as these fluctuations in potential are commonly referred to) are in the millivolt {a thousandth of a volt) and microvolt f a millionth of a volt) ranges. At the very beginning of the history of EEG, Berger noticed that the potentials fluctuated in rhythmic fashion, though the rhythm was not a simple one, but made up of several types of contributory waves.

Berger gave the most pronounced rhythm the name of alpha wave. In the alpha wave the potential varies by about 20 microvolts in a cycle of roughly 10 times a second. The alpha wave is clearest and most obvious when the subject is resting with his eyes closed. At first Berger's suggestion that the brain as a whole gives rise to this rhythm seemed acceptable. Since his time increasingly refined investigation has altered things. More and more electrodes have been applied to the skull in various places (the positions kept symmetrical about the midplane of of the skull) and now as many as twenty-four places may be tapped and the potential differences across a number of these can be recorded simultaneously. In this way it has been discovered that the alpha wave is strongest in the occipital region

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of the skull, or, to say it differently, in the area where the visual center is located.

When the eyes are opened, but are viewing featureless illumination, the alpha wave persists. If, however, the ordinary variegated environment is in view, the alpha wave vanishes, or is drowned by other, more prominent rhythms. After a while, if nothing visually new is presented, the alpha wave reappears. It is possible that the alpha wave represents the state of readiness in which the visual area holds itself when it is being only minimally stimulated. (It would be almost like a person shifting from foot to foot or drumming his fingers on the table as he waits for some word that will rouse him to activity.) Since sight is our chief sense and provides us with more information than any other sense does, and is therefore the chief single factor in keeping the brain busy, it is not surprising that the alpha wave dominates the resting EEG. When the eyes actually begin reporting information and the nerve cells of the cortex go to work on it, the "waiting" rhythm vanishes. If the visual pattern remains unchanged so that the brain eventually exhausts its meaning, the "waiting" rhythm returns. The brain cannot "wait" indefinitely, however. If human beings are kept for long periods without sensory stimulation, they undergo difficulties in trying to think or concentrate and may even begin to suffer from hallucinations (as though the brain in default of real information begins to make up its own). Experiments reported in 1963 indicated that men kept in environments lacking sensory stimulation for two weeks showed progressively smaller alpha waves appearing in the EEG.

In addition to alpha waves, there are also beta waves, representing a faster cycle, from 14 to 50 per second, and a smaller fluctuation in potential. Then, too, there are slow, large delta waves and rather uncommon theta waves.

The EEG presents physiologists with reams of fascinating data, most of which they are as yet helpless to interpret. For instance, there are differences with age. Brain waves can be detected in the fetus, though they are then of very low voltage and very slow.

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They change progressively but do not become fully adult in characteristics till the age of  17.   There  are  also changes  in brain waves characteristic of the various stages of falling asleep and waking up, including changes when, presumably, the subject is dreaming.   (Delta waves accompanied by rapid movements of the eyes are prominent during these dream intervals.) In contrast to all this variegation, the EEC of the various animals tested are quite similar in general characteristics among themselves and when compared with those of man.   The brain, of whatever species, seems to have but one basic fashion of operation. As far as analyzing the EEC is concerned, we might make an analogy with a situation whereby all the people on earth are listened to from a point out in space.   It might be possible to detect human noise as a large buzz with periodic tiny irregularities (representing the passage of rush-hour traffic, evening hilarity, nighttime sleep, and so on) progressing around the world. To try to get information from the EEC as to the fine details of behavior within the cortex would be something like trying to analyze the overall buzz of the world's people in order to make out particular conversations.

Specially designed computers are now being called into battle. If a particular small environmental change is applied to a subject, it is to be presumed that there will be some response in the brain that will reflect itself in a small alteration in the EEG pattern at the moment when the change is introduced. The brain will be engaged in many other activities, however, and the small alteration in EEG will not be noticeable against the many complicated wave formations. Notwithstanding, if the process is repeated over and over again, a computer can be programed to average out the EEG pattern and compare the situation at the moment of environmental change against that average. In the long run there would be, it is presumed, a consistent difference. Yet there are times when the EEG has diagnostic value in medicine even without the refined work of the latest computers. Naturally, this is so only when the EEG is radically abnormal

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and therefore when the brain is suffering from some serious malfunction. (Thus, the hypothetical listener to the overall buzz of the world's people might be able to detect a war. and even locate its center of action, by the unusual sound of artillery rising above the ordinary melange of sound.)

One case in which EEG is useful is in the detection of brain tumors. The tissue making up the tumor itself is not functional and delivers no brain waves. The areas of the cortex immediately adjacent to the tumor deliver distorted brain waves. If enough EEG records are taken over enough areas of the brain and if these are subjected to careful enough analysis, it is possible sometimes not only to detect the existence of a tumor but even to locate its position on the cortex. However, it will not detect a tumor in any part of the brain but the cerebral cortex.

The EEG is also useful in connection with epilepsy ("seizure" G), so called for reasons explained below. Epilepsy is a condition in which brain cells fire off at unpredictable moments and without the normal stimulus. This may be due to damage to the brain during prenatal life or during infancy. Often it has no known cause. The most dramatic form of such a disease is one where the motor area is affected. With the motor nerves stimulating muscles randomly, the epileptic may cry out as the muscles of the chest and throat contract, fall as the muscular coordination controlling balance is disrupted, and writhe convulsively. The fit doesn't last long, usually only a few minutes, but the patient can do serious damage to himself in that interval. Such fits, at unpredictable intervals, are referred to as grand mal (grahn mal; "great sickness," French). A more direct English name is "falling sickness."

In another version of epilepsy, it is the sensory area that is primarily affected. Here the epileptic may suffer momentary hallucinations and have brief lapses of unconsciousness. This is petit mal (puh-tee mal; "small sickness" French). Both areas may be mildly affected, so that a patient may have illusions followed by disorganized movements. These are psychomotor attacks.

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Epilepsy is not very uncommon, since about i in 200 suffer from it, though not necessarily in its. extreme form. It has a fascinating history. Attacks of grand mal are frightening and impressive, particularly in primitive societies (and some not so primitive) that do not understand what is taking place. During the attack, the epileptic's muscles are clearly not under his own control and it is easy to conclude that his body has been momentarily seized by some supernatural being. (Hence, the fit is considered a "seizure" and thus the name "epilepsy" arises.) Some famous people, including Julius Caesar and Dostoyevsky, have been epileptics.

The supernatural being may be conceived of as an evil demon, and the existence of epileptic seizures are partly to blame for the belief, even down to modern times, in demonic possession. The epileptic may be felt to have gained supernatural insights into the future as a result of his intimate relationship with the supernatural. A Delphic prophetess was always more impressive if she experienced (or counterfeited) an epileptic fit before delivering her prophecy. Modern mediums, during spiritualistic seances, are careful to writhe convulsively. To the Greeks epilepsy was "the sacred disease." Hippocrates, the "father of medicine" (or possibly some disciple), was the first to maintain that epilepsy was a disease like other diseases, caused by some organic failing, and potentially curable without recourse to magic.

The EEG is characteristic for each variety of epilepsy. Grand mal shows a pattern of high-voltage fast waves; petit mal, fast waves with every other one a sharp spike; psychomotor attacks, slow waves interspersed by spikes. The brain-wave pattern can be used to detect subclinical attacks that are too minor to be noticed otherwise. It can also be used to follow the reaction of patients to treatment by noticing the frequency and extent of these abnormal patterns.

Other uses of EEG are in the process of being developed. Thus, the brain, because of its critical need for oxygen and glucose, is the first organ to lose function in a dying patient. With

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modern techniques of resuscitation, it is not impossible that patients may be revived while the heart is still beating but after the higher centers of the brain are irretrievably gone. Life under those conditions is scarcely to be called life, and it has been suggested that loss of EEG rhythms be considered as marking death even though the heart is still struggling to beat.

EEG may be useful in diagnosing and even in learning to understand various psychotic states, which is something I shall come back to in Chapter 14.

THE   BASAL   GANGLIA

The portion of the cerebrum beiow the cortex is, as mentioned earlier in the chapter, largely white matter, made up of myelin-sheathed nerve fibers. Just above the various ventricles making up the hollow within the brain, for example, is a tough bridge of white matter, the corpus callosum (kawr'pus ka-loh'sum; "hard body" L), which binds the two cerebral hemispheres together (see illustration, p. 165). Nerve fibers cross the corpus callosum and keep the cerebrum acting as a unit, but in some ways the hemispheres are, at least potentially, independent.

The situation is somewhat analogous to that of our eyes. We have two eyes which ordinarily act as a unit. Nevertheless, if we cover one eye we can see well enough with the remaining eye; a one-eyed man is not blind in any sense of the word. Similarly, the removal of one of the cerebral hemispheres does not make an experimental animal brainless. The remaining hemisphere learns to carry on. Ordinarily each hemisphere is largely responsible for a particular side of the body. If both hemispheres are left in place and the corpus callosum is cut, coordination is lost and the two body halves come under more or less independent control. A literal case of "twin brains" is set up. Monkeys can be so treated (with further operation upon the optic nerve in order to make sure that each eye is connected to only one hemisphere), and when this is done each eye can be separately trained to do par-

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ticular tasks. A monkey can be trained to select a cross over a circle as marking, let us say, the presence of food. If only the left eye is kept uncovered during the training period, only the left eye will be useful in this respect. If the right eye is uncovered and the left eye covered, the monkey will have no right-eye memory of his training. He will have to hunt for his food by trial and error. If the two eyes are trained to contradictory tasks and if both are then uncovered, the monkey alternates activities, as the hemispheres politely take their turns.

Naturally, in any such two-in-charge situation, there is always the danger of conflict and confusion. To avoid that, one cerebral hemisphere (almost always the left one in human beings) is dominant. Broca's convolution, see page 173, which controls speech, is in the left cerebral hemisphere, not the right. Again, the left cerebral hemisphere controls the motor activity of the right-hand side of the body, which may account for the fact that most people are right-handed (though even left-handed people usually have a dominant left cerebral hemisphere). Ambidextrous people, who may have cerebral hemispheres without clear-cut dominance, sometimes have speech difficulties in early life.

The subcortical portions of the cerebrum are not all white matter. There are collections of gray matter, too, below the cortex. These are called the basal ganglia.' The piece of gray matter that lies highest in the cerebral interior is the caudate nucleus ("tailed" L, because of its shape). The gray matter of the caudate nucleus bends upon itself, and its other end is the amygdaloid nucleus ("almond-shaped," again because of its shape). To one side of the caudate nucleus is the lentiform nucleus ("lens-shaped" ) and between the two is the white matter of the internal capsule. The nuclei are not uniformly gray but contain white matter with fibers of gray matter running through it and giving

•The word ganglion {ganglee-on) is (keek for "knot" and was originally used by Hippocrates and his school for knotlike tumors beneath the skin. Galen, the Roman physician who flourished about A.D. 200, began to use the word for collections of nerve cells which stood out like knots against the ordinarily string-like nerves, and it is still so used.

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the region a striated appearance. The region including the two nuclei is therefore called the corpus striatum.

Within the curve of the caudate-nucleus, corpus-striatum, and lentiform-nucleus complex lies a mass of gray matter that represents the thalamus. (The reason for the name is given on page 145.) The thalamus is usually not included among the basal ganglia, though it is right there physically.

The basal ganglia are difficult to study, obscured as they are by the cerebral cortex. Yet there are indications of importance both in the passive and active sense. The white matter of the corpus striatum is a bottleneck. All motor nerve fibers descending from the cortex and all sensory fibers ascending up toward it must pass through it. Consequently, any damage to that region can have the most widespread effects. Such damage can, for instance, lead to loss of sensation and capacity for motion in a whole side of the body; the side opposite to that of the cerebral hemisphere within which the damage took place. Such one-sided loss is called hemiplegia (hem-ee-plee'jee-uh; "half stroke" G).°

It has been suggested that one of the functions of the basal ganglia is to exert a control over the motor area of the cortex (by way of the extra-pyramidal system of which it forms a part) and to prevent the motor area from kicking off too readily. When this function of the basal ganglia is interfered with, sections of the motor area may indeed fire off too readily, and then there are rapid involuntary muscle contractions. The muscles usually affected in this way are those controlling the head and the hands and fingers. As a result, the head and hand shake continuously and gently. This shaking is most marked when the patient is at rest and smooths out when a purposeful motion is superimposed; in other words, when the motor area goes into real action instead of leaking slightly.

* The loss of the capacity for motion is referred to as paralysis from a Greek word meaning "to loosen." The muscles fall loose, so to speak. Conditions thai bring on sudden paralysis, as the rupture of a blood vessel in the brain, is referred to as a "stroke" both in plain English and in the Greek, because the person is felled as though struck by a blunt instrument.

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Other muscles become abnormally immobile in such cases, although there is no true paralysis. The facial expression becomes comparatively unchanging and masklike; walking becomes stiff, and the arms remain motionless instead of swinging naturally with the stride, This combination of too little movement in arms and face and too much movement in head and hands receives the self-contradictory name of paralysis agitans (aj'ih-tans; "to move" L). The self-contradictoriness is maintained in the English name of "shaking palsy." (The word "palsy" is a shortened and distorted form, descended through medieval French, of "paralysis.") Paralysis agitans was first clinically described in 1817 by an English physician named James Parkinson; it is also commonly known as Parkinson's disease.

Relief from some of the symptoms has been achieved by deliberately damaging the basal ganglia, which seems to be a case of "a hair of the dog." One technique is to locate the abnormal region by noting at which point a touch of a thin probe wipes out, or at least decreases, the tremor and rigidity, and then cooling the area to —50° C. with liquid nitrogen. This can be repeated if the symptoms recur. Apparently, nonfunctioning ganglia are preferable to misfunctioning ones.

Sometimes damage to the basal ganglia may result in more spasmodic and extensive involuntary muscular movements; almost as though a person were dancing in a clumsy and jerky manner. Such movements are referred to as chorea (koh-ree'ah; "dance" G). This may strike children as an aftermath of rheumatic fever, where the infection has managed to involve the brain. An English physician named Thomas Sydenham first described this form of the disease in 1686, so that it is usually called Sydenham's chorea.

During the Middle Ages there were instances of "dancing manias" that swept over large areas. These probably were not true chorea epidemics, but had roots in abnormal psychology. It is possible, however, that specific cases of mania may have been set off by a true case of chorea. Someone else might have fallen

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into line in hysterical imitationl others followed, and a mania was under way. It was felt at one time that to be cured of the dancing mania one ought to make a pilgrimage to the shrine of St. Vitus. It is for this reason that Sydenham's chorea has the common name of "St. Vitus's dance."

There is also hereditary chorea, often referred to as Hunting-ton's chorea, from the American physician George Sumner Hun-tington, who described it in 1872. It is much more serious than the St. Vitus's dance, from which, after all, one recovers. Hun-tington's chorea does not appear until adult life (between 30 and 50). Mental disorders accompany it; it grows steadily worse, and is eventually fatal. It is an inherited condition, as one of its names implies. Two brothers afflicted with the disease migrated to the United States from England, and all modern American patients are supposed to be descended from them.

The thalamus acts as a reception center for the somesthetic sensations — touch, pain, heat, cold, and the muscle senses. It is indeed an important portion of the reticular activating system which accepts and sifts incoming sensory data. The more violent of these, such as pain, extreme heat or cold, rough touch, are filtered out. The milder sensations from the muscles, the gentle touches, the moderate temperatures are passed on to the sensory area of the cortex. It is as though mild sensations can be trusted to the cerebral cortex, where they can be considered judiciously and where reaction can come after a more or less prolonged interval of consideration. The rough sensations, however, which must be dealt with quickly and for which there is no time for consideration, are handled more or less automatically in the thalamus.

There is a tendency, therefore, to differentiate between the cortex as the cold center of reason and the thalamus as the hot focus of emotion. And, as a matter of fact, the thalamus controls the movement of facial muscles under conditions of emotional stress, so that even when cortical control of those same muscles is destroyed and the expression is masklike in calm states, the face can still twist and distort in response to strong emotion. In

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addition, animals in which the cortex is removed fall easily into all the movements associated with extreme rage. Despite the foregoing, this sharp distinction between cortex and thalamus would seem to be an oversimplification. Emotions do not arise from any one small part of the brain, it would appear. Rather, many parts, including the frontal and temporal lobes of the cortex, are involved—in a complex interplay. Removing the temporal lobes of animals may reduce their emotional displays to a minimum even though the thalamus is not affected.

In recent years, attention has been focused on certain portions of the cerebrum—old portions, evolutionarily speaking, related to ancient olfactory regions—which are particularly associated with emotion and with emotion-provoking stimuli such as hunger and sex. This region seems to coordinate sensory data with bodily needs, with the requirements of the viscera, in other words. It is therefore referred to as the visceral brain. The convolutions associated with the visceral brain were named the limbic lobe ("border" L) by Broca because they surrounded and bordered on the corpus callosum. For this reason, the visceral brain is sometimes called the limbic system.

THE    HYPOTHALAMUS

Underneath the third ventricle and, consequently, underneath the thalamus, is the hypothalamus ("beneath the thalamus" G), which has a variety of devices for controlling the body. Among the most recently discovered is a region within it which on stimulation gives rise to a strongly pleasurable sensation. An electrode affixed to the "pleasure center" of a rat, so arranged that it can be stimulated by the animal itself, will be stimulated for hours or days at a time, to the exclusion of food, sex, and sleep. Evidently all the desirable things in life are desirable only insofar as they stimulate the pleasure center. To stimulate it directly makes all else unnecessary. (The possibilities that arise in connection with a kind of addiction to end all addictions are distressing to contemplate.) Because of the several ways in which the hypothalamus sets

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up automatic controls of bodily functions, one can look upon it as having functions not very different from those of sets of hormones acting in cooperating antagonism (such as insulin and glucagon, for instance). There is, in reality, a physical connection between the hypothalamus and the world of hormones as well as a vague functional one. It is to the hypothalamus that the pituitary gland is attached, and the posterior lobe of the pituitary actually arises from the hypothalamus in the course of its development.

It is not surprising, then, that the hypothalamus is involved in the control of water metabolism in the body. I have already described how the posterior pituitary controls the water concentration in the body by regulating reabsorption of water in the kidney tubules. Well, it would seem that one can go a step beyond the posterior pituitary to the hypothalamus. Changes in the water concentration in the blood stimulate a particular hypothalamic center first, and it is the hypothalamus that then sets off the posterior pituitary. If the stalk connecting the hypothalamus to the posterior pituitary is cut, diabetes insipidus results, even though the gland itself remains unharmed. Some recent experiments suggest the hypothalamns may stimulate the anterior pituitary as well; in the production of ACTH, for instance.

The hypothalamus also contains a group of cells that acts as a very efficient thermostat. We are conscious of temperature changes, of course, and will attempt to rectify extremes by adding or subtracting clothing and by the use of furnaces and air-conditioners. The hypothalamus reacts analogously, but much more delicately and with built-in devices.