The human brain has been characterized as “nature’s master contrivance,” yet it must be remembered that the great majority of organisms on the earth fill out their cycle of existence, accomplishing successfully the two major activities of self maintenance and the continuation of their kind without the aid of so elaborate a mechanism as that which in man becomes the “most complicated organ known” (Minot). Nevertheless with a human brain it becomes possible to live a human life, which involves something more than metabolism and reproduction. This lofty goal has been gained, as comparative anatomy reveals, only after much evolutionary travail.
The vertebrate brain is the enlarged anterior end of the medullary tube. Like the cord it is a continuous tubular structure, the walls of which are composed of neurons that are arranged in communicating tracts and adjusting centers, supported and protected by non-nervous elements.
The brain became differentiated from the cord as the result of a mobilization of nerve elements in the neighborhood of the important sense organs of smell, sight, hearing, and taste, which are on guard near the entrance of the digestive canal. Whenever these sense organs are reduced or absent, as in deep-sea or cave animals, or in sessile forms, the brain diminishes in corresponding degree. Although the sense organs of the head unquestionably take a prominent part in determining the modifications which the brain undergoes, they by no means set the limit to its differentiation, for there are many animals with inferior brains whose sense organs are superior to those of man. It is primarily the associative centers that form the outstanding feature of the brain. Its general mass and contour, unlike that of the cord with its brachial and lumbar enlargements, are not solely dependent upon the area of stimulus-receiving surface involved, or the amount of glandular tissue supplied.
In other words, the conspicuous enlargements that characterize the walls of the brain are not so much due to incoming and outgoing limbs of reflex arcs that relate the animal directly to its environment, as to internal associative connections which, at least in man, enable the brain which “sits in darkness” not only to reconstruct the outside world but also to preserve the accumulations of memory and even to provide for creative flights of imagination.
The primary axis, or brain stem, is directly concerned with peripheral nerve relations, the motor centers and pathways, both somatic and visceral, being located along the ventral region as in the cord, while the visceral and somatic sensory neurons occupy the dorsal area (Fig. 627). Upon this primary cordlike axis of simple reflex-arc systems, is superimposed the relaying association centers of correlation and coordination, the complex cerebrum and cerebellum, which are once removed from direct sense reception and muscular response. Correlation centers concern the adjustment of incoming sensory stimuli, while coordination centers regulate the outgoing motor responses. These superimposed centers of the brain may be compared to the switchboard of a telephone exchange, where messages are received and distributed.
Comparison of Brain and Cord
The central nervous system of amphioxus is all cord and no brain (Fig. 628). The true rise of the vertebrate brain begins in fishes, where the cord still outweighs the brain. Among amphibians the brain overtakes the cord in bulk and weight, while in modern reptiles it gets the lead, which ever afterwards in the vertebrate series is maintained.
In mammals there is an ascending dominance of the brain over the cord. A cat and a macaque monkey, for example, with cords of the same weight (7.5 grams), have been found to have brains weighing 29 and 62 grams respectively, or ratios of 1:4 and 1:8. The ratio in man is nearer 1:50, an average measurement being 26 grams for the cord as compared with 1350 grams for the brain.
The absolute weight of the brain, it should be noted, is not in itself a reliable criterion of intelligence, since various factors, such as age, sex, form of skull, and weight of body, as well as the comparative size of different parts of the brain, must be taken into account. It is quality rather than quantity that counts.
The average human male has a brain of about 1350 grams in weight, while that of the average human female, not to start an argument but to state a fact, is something like 100 grams less. Sometimes the size of a brain is determined more by the non-nervous neuroglia cells than by the neurons.
Some notable deviations from the average weight of human brains are recorded as follows: Thackeray, the novelist, 1644; Cuvier, the comparative anatomist, 1830; Turgenieff, the historian,' 2012; Haeckel, the biologist, 1575; Agassiz, the zoologist, 1495; Schumann, the musician, 1475; Gambetta, the statesman, 1294; Whitman, the poet, 1282; Bollinger, the anatomist, 1207; and Anatole France, the writer, 1017. It is not the size of your wrist watch that is of importance, but how good time it keeps.
The weight of the brain of a dog, gorilla, and man, having approximately the same weight of body, has been found to be 135, 430, and 1350 grams respectively.
The actual mass of the human brain is exceeded among animals only by that of the gigantic elephants and whales, the size of whose body is many times that of man. An examination of the cranial cavities of the great reptiles of the Mesozoic age reveals the fact that they had insignificant brains in proportion to the enormous bulk of their bodies. Indeed it is a source of astonishment that these monsters of the past were able to get about with brains relatively so small, but it must be remembered that eventually, after an evolutionary experiment of several million years, they did succumb. It is doubtless more than a coincidence that the twilight of the dinosaurs and their reptilian contemporaries fell at about the same time as the dawn of the mammals who were at first comparatively insignificant in size, but who had a new and revolutionary ratio established between weight of brain and weight of body.
Differentiation of the Brain
The transforming embryonic brain is a key to the adult structure. The modification of the simple tubular brain into the exceedingly complex structure found in man is brought about through the following phases of growth: (a) constriction; (b) unequal thickenings of the walls; (c) invagination and evagination; and (d) bending.
The original, somewhat inflated anterior region of the medullary tube, which is formed upon the closure of the medullary groove, is called the encephalon (Fig. 629A).
Very early in embryonic life (the third week in man), the encephalon becomes marked off into three regions by two constrictions (Fig. 629b). These primitive regions are designated as the prosencephalon, mesencephalon, and rhombencephalon, the latter so called because of the kite-shaped appearance of the ventricle, or cavity, showing here.
A little later (the fourth week in man), the prosencephalon becomes further constricted into two subregions, the telencephalon and the diencephalon (Fig. 629c). The rhombencephalon likewise (about the fifth week in man) is subdivided into a metencephalon and a myelencephalon, the mesencephalon remaining unconstricted as it was before. The brain is now marked off into five definite areas (Fig. 629d), arranged in order from anterior to posterior as follows: telencephalon, diencephalon, mesencephalon, metencephalon, and myelencephalon.
Certain of these primary regions of the brain are destined to take on special importance, because of the sense organs with which they are intimately connected. The telencephalon may be regarded as the “nose brain,” the mesencephalon as the “eye brain,” and the myelencephalon as the “ear brain,” with the additional assignment to the latter of “skin brain” and “visceral brain.”
Cyclostomes throughout life remain with the brain constricted into only three regions, but from fishes onward the five fundamental parts of the brain are clearly represented.
Unequal Thickening of the Walls of the Brain
The walls of the primitive encephalon are at first of approximately equal thickness, but as neurons develop and collect into centers and transmission tracts, the disposition of these collections of nerve elements brings about decided contrasts in the thickness of the walls of the brain in different places.
The thinnest regions are at two points, the roofs, or vela, (1) of the diencephalon and small median portion of the telencephalon and (2) of the myelencephalon (Fig. 630). Each velum includes only the thin ependymal layer.
The thickest areas, on the other hand, are the cerebellum, which develops from the dorsal part of the metencephalon, and the walls of the cerebral hemispheres, the paired antero-lateral outgrowths which make up most of the telencephalon. The cerebral hemispheres, the crowning hope of the evolving brain, become so enormously thickened and enlarged in mammals that they overshadow and entirely cover up both the diencephalon and the mesencephalon, concealing them from dorsal view.
Invaginations and Evaginations
There are two places where the brain wall is pushed in or invaginated, namely, in the regions of the thin vela just mentioned. Over the outside of these thinnest places in the walls of the brain extends a network of capillaries in the pia mater, the vascular meninx that intimately envelops the brain. These capillary nets are the choroid plexuses, which by invagination are pushed down into the cavities of the brain below, carrying the thin wall with them. They provide a blood supply for the inside of the brain.
From the walls of the diencephalon, there also occur various evaginations. In certain vertebrates these are two finger-like outpushings of the wall, one behind the other, on the dorsal side just behind the invaginated choroid plexus. The anterior of these evaginations, near the junction of the cerebral hemispheres and the diencephalon, is the parietal organ, which is particularly developed in certain reptiles, while the posterior projection, the epiphysis, or pineal “gland,” appears in some stage of degeneration or elaboration in practically all vertebrates (Fig. 630).
A ventral median evagination of the floor of the diencephalon, the infundibulum, has already been mentioned. It joins a glandular upgrowth from the roof of the mouth to form the hypophysis, which, like the pineal body, has been briefly considered in Chapter XVI as a gland of internal secretion.
The optic stalks extending out from the sides of the diencephalon in early embryonic life are also evaginations (Fig. 631).
In cyclostomes and other lower vertebrates the parts of the brain retain their primitive tandem arrangement, but with the onrush of growth and the ultimate confining limits of the skull, it becomes necessary for the brain stem to bend and fold to accommodate itself to more compact quarters. The bends, or flexures, of the brain are typically three: (1) the parietal flexure in the region of the mesencephalon; (2) the cervical flexure near the junction of the medulla and the cord, and (3) the pontine flexure between these two. The first two flexures bend ventrally while the third bends in the opposite direction, thus tending to kink the brain stem together like a compressed accordion (Fig. 632).
The central canal, within the cord, remains very small and is approximately uniform in size throughout its length. It is the result of invagination and growth. Continuous with the central canal, and like it filled with cerebro-spinal fluid, are the cavities or ventricles of the brain (Fig. 631). These are chambers of unequal dimensions, and are frequently encroached upon but never entirely obliterated by the thickening walls of the brain itself.
The fourth ventricle is the most posterior chamber, located in the myelencephalon and metencephalon. It is roofed over by the thin velum which in ordinary dissection is frequently torn away, exposing this ventricle as a somewhat triangular trough, narrowing posteriorly to become the central canal. Anteriorly the fourth ventricle leads into the cavity of the mesencephalon, a large mesocoele in lower vertebrates but reduced to a slender canal, the aqueduct of Sylvius, when the walls of this region thicken in most higher groups.
The aqueduct communicates anteriorly1 with a narrow vertical slit-like third ventricle, the cavity of the diencephalon and small median portion of the telencephalon. Its anterior wall is the lamina terminalis, the anterior end of the embryonic brain, while its roof is the thin velum. The paired cerebral hemispheres, which grow laterally from the telencephalon, enclose paired cavities, the lateral ventricles. Each of these lateral cavities communicates with the small telencephalic part of the third ventricle through an interventricular foramen, sometimes called the foramen of Monro.
General Topography of the Mammalian Brain
Before considering the chief evolutionary changes in the brains of vertebrates, we may first discuss the main features of the mammalian brain, which represents the last chapter in the story but which has been more thoroughly studied than that of other animals.
The dividing point between the spinal cord and the brain is a somewhat arbitrary one, there being no sharp line of demarcation between the cord and the posterior part of the myelencephalon, or medulla oblongata. It is usually stated that the brain is that part of the central nervous system which is within the skull, while the cord begins at the foramen magnum, at a point immediately anterior to the first cervical spinal nerve.
Although the posterior end of the human medulla oblongata resembles the cord in both external appearance and internal features, its anterior portion differs considerably from the cord, as a result of many rearrangements of parts here. The dorsal fissure continues forward to about the middle of the medulla where the spreading of its lateral walls, the funiculi gracilis and cuneatus, transforms the roof-plate region into a broad thin velum interpositum (Fig. 633). The central canal, which has continued into the medulla, expands into the fourth ventricle as another result of the spreading of these funiculi which thus become the lateral walls of this cavity (Fig. 634). The thin velum is covered by a highly vascular pia mater from which tufts of blood vessels push into the fourth ventricle, carrying the velum ahead of them, to form the posterior choroid plexus. In each of the lateral walls of the ventricle, fibers continue forward to the anterior end of the medulla and then turn dorsally to enter the cerebellum. This pair of connections, known as the restiform bodies, or posterior cerebellar peduncles, is the chief pathway joining the cerebellum with the medulla and cord.
The ventral fissure terminates at the anterior end of the medulla (Fig. 635). Anteriorly this fissure separates two longitudinal swellings, the pyramids, lying side by side in the ventral part of the medulla. These structures are composed largely of the corticospinal tracts that extend from the gray matter of the cerebrum down into the cord. Near the posterior end of the medulla, bundles of pyramidal fibers cross obliquely from one side to the other, forming the pyramidal decussation (decuss-, to cross as in an X) which interrupts the ventral fissure. The fibers which cross at this point make up the crossed pyramidal tract of the cord. In man about one-fourth of the pyramidal fibers do not cross but continue into the cord as the direct pyramidal tract.
Eight pairs of cranial nerves, V-XII inclusive, connect with the mammalian medulla.
Anterior to the medulla and surrounding the anterior portion of the fourth ventricle is the metencephalon. The dorsal part of this region is greatly enlarged into the cerebellum, a structure which has been called the “gyroscope of the body,” because it is chiefly concerned with muscular coordination and maintenance of equilibrium. The cerebellum consists of a median vermis, so named because of its wormlike appearance, on either side of which is a large cerebellar hemisphere. Within the cerebellum, the gray matter forms an outer layer, the cerebellar cortex, covering over the inner white matter. In longitudinal sections, particularly those through the vermis, the white matter shows a conspicuous tree-like arrangement to which the name arbor vitae has been applied (Fig. 634).
Perhaps the most interesting cells in the entire brain are the Purkinje cells (Fig. 636), abundant in the outer part of the cerebellar cortex. Each one has two or three dendrites which branch repeatedly to form hundreds of minute fibers spreading out chiefly in one plane “like the branches of a vine on a trellis,” to use Ranson’s apt comparison. Near the cell body the neurite, which leads toward the center of the cerebellum, gives off collaterals that run back toward the surface of the cortex to synapse with dendrites of other Purkinje cells, thus linking together many of these elements. These cells are of particular interest because of the part they presumably play in the coordination of the muscles.
The ventral portion of the metencephalon is the pons, so named because its ventral or outer part is made up of a broad “bridge” of fibers looping around between the two cerebellar hemispheres (Fig. 635). In the deeper or inner part of the pons are found most of the tracts represented in the anterior part of the medulla.
The cerebellum is connected with the other parts of the brain by three pairs of structures known as cerebellar peduncles. The posterior cerebellar peduncles were described above as the restiform bodies which connect medulla and cerebellum (Fig. 633). The middle cerebellar peduncles, the largest of the three pairs in man, pass around the sides of the brain to join the pons. The anterior peduncles of the cerebellum run forward into the mesencephalon. Between these last named structures the roof of the fourth ventricle is again merely a thin layer of ependymal cells. This thin roof, extending from the white matter of the cerebellum forward to the roof of the mesencephalon, is known as the anterior medullary velum (Figs. 633 and 637). No choroid plexus is associated with this velum.
The mesencephalon, or mid-brain, is the most conservative region of the brain. Its walls, of moderate thickness, surround the slender aqueduct which connects the third and fourth ventricles (Fig. 637). The dorsal side of this region is marked by four rounded elevations, the corpora quadrigemina, arranged one pair behind the other (Figs. 633 and 637). The anterior ones, or anterior colliculi, are important reflex centers associated with visual impulses, while the posterior colliculi are auditory centers. The ventral surface is raised up into a pair of conspicuous rounded ridges, the cerebral peduncles or crura cerebri, arranged like a V with their converging posterior ends passing into the pons (Fig. 635). More than half of the fibers which make up these crura belong to the corticospinal tracts. Dorsal to each cerebral peduncle the ventro-lateral part of the mesencephalon is known as the tegmentum, a region composed of various tracts and nuclei, including the nuclei of cranial nerves III and IV.
The diencephalon, which encloses all except the most anterior portion of the slit-like third ventricle, is a small but complex region composed of the following parts: (1) the velum interpositum of the third ventricle, a thin non-nervous portion of the roof-plate; (2) the epithalamus, the nervous portion of the dorsal region; (3) the thalamus, a large mass on each side forming most of the lateral wall of the ventricle; and (4) the hypothalamus, or ventral part.
The velum forms most of the roof of the third ventricle including the extreme anterior part which lies in the small median portion of the telencephalon (Fig. 637). It is covered by the vascular pia mater which sends capillary masses into the third ventricle, pushing the ependymal layer of the velum ahead of them. These capillaries, the anterior choroid plexuses, are not confined to the third ventricle but also extend through the interventricular foramina into the lateral ventricles of the telencephalon.
The epithalamus, the somewhat U-shaped remainder of the roof, includes the pineal body and the habenula, both of which lie just in front of the anterior colliculi, at the closed end of the U (Fig. 633). The habenula is made up of a pair of habenular ganglia connected by a bundle of fibers, the habenular commissure, which forms the transverse part of the “U”. The habenula is an olfactory center found in all vertebrates. The pineal body, a small pine-cone-shaped structure attached by a short stalk to the habenular commissure, has already been considered in Chapter XVI in connection with the ductless glands.
Each thalamus is a large oblong mass including many centers, as well as bundles of fibers connecting the cerebral hemispheres with the remainder of the brain and the cord. The crura cerebri, running obliquely forward and outward to enter the cerebral hemispheres, skirt the ventro-lateral border of the thalamus.
The hypothalamus, lying between the spreading anterior ends of the crura cerebri, forms the floor and part of the lateral wall of the third ventricle. It includes the optic chiasma, tuber cinereum, mammillary bodies, infundibulum, and posterior lobe of the hypophysis. The optic chiasma, where fibers of the optic nerves (Cranial II) cross, is at the anterior end of the hypothalamus (Figs. 635 and 637). Immediately posterior to the chiasma is the tuber cinereum, an elevation to which the hypophysis (see Chapter XVI) is attached by the funnel-shaped infundibulum. The mammillary bodies, a pair of small rounded elevations in the posterior part of the base of the tuber cinereum, are olfactory centers.
The telencephalon includes a small median portion and two large cerebral hemispheres which arise embryonically as lateral evaginations of the median region. In the median part is found the extreme anterior end of the third ventricle from each side of which an interventricular foramen, or foramen of Monro, opens into the lateral ventricle of the corresponding hemisphere. The anterior wrall of the third ventricle is a thin membrane, the lamina terminalis, which marks the anteriormost part of the brain in the mid-line (Fig. 637).
Each cerebral hemisphere differentiates very early into three regions, corpus striatum, olfactory lobe, and pallium. The corpus striatum is the thick ventro-lateral part of the hemisphere. The olfactory lobe, or rhinencephalon (rhin, nose), evaginates from the cerebral floor anterior to the corpus striatum. The remainder of the hemisphere is the thin-walled pallium. As the mammalian pallium grows much more rapidly than the other two parts, it soon makes up most of the hemisphere, extending both anterior and posterior to the other parts as well as expanding laterally and dorsally until it meets the pallium of the other side, dorsal and also anterior to the diencephalon. Between the enlarged hemispheres there persists a narrow space, known as the longitudinal cerebral fissure, in which lie folds of the meninges of the brain.
The corpus striatum is so named because in the adult it consists of gray nuclear masses alternating with sheets of white medullated fibers. According to Ranson little is known about the function of this region.
The pallium includes an outer gray layer, the cerebral cortex, and an inner complex of white matter. In mammals only a small portion of the pallium, known as the archipallium, is associated with olfactory impulses although in lower vertebrates virtually the entire telencephalon has this function. In the cat, the archipallium shows on the surface of the hemispheres only as the ventral pyriform lobe with which are connected the olfactory tract and olfactory bulb (Fig. 638). The olfactory nerves (Cranial I) lead from sensory endings in the nasal cavity to the olfactory bulbs. In man the archipallium is even more restricted than in carnivores.
The great non-olfactory portion of the pallium, known as the neopallium, is the main center of the whole central nervous system in mammals. In the cerebral cortex of the neopallium are located the centers of conscious sensations and of voluntary motor control. White fibers immediately beneath the outside gray layer form an intramural network of great complexity, connecting different regions of the cortical area with one another and with other parts of the brain forming a unifying and integrating system of supreme importance.
The two hemispheres become secondarily joined by commissures, transverse bands of medullated fibers. The anterior commissure, developing in the lamina terminalis, connects the olfactory, or archipallial, regions. The corpus callosum, a broad band which spreads out extensively in the hemispheres, establishes elaborate connections between their neopallial portions. Both of these parts may be seen in sagittal sections of the brain (Figs. 634 and 637). Two other portions of the telencephalon which show clearly in such sections are the fornix and the septum pellucidum. The fornix is a curved longitudinal band of white fibers of olfactory function. The septum pellucidum, between the fornix and the corpus callosum, is a thin median wall separating the lateral ventricles. Between the two layers of the septum is a cavity which has been called the “fifth ventricle,” a term which is unfortunate in view of the fact that it is not derived from the cavity of the embryonic medullary tube and hence is in no way comparable to the true ventricles of the brain.
In many mammals, particularly the larger ones, the cortical surface of the cerebral hemispheres is increased by the formation of folds or convolutions, not found in monotremes, marsupials, insectivores, bats, and most rodents. The miniature mountain chains thus formed are called gyri, and the valleys between them, sulci (Fig. 639), while particularly deep sulci are designated as fissures. Mammals with convoluted brains have the gyri and sulci already marked out at birth. Quite contrary to prevalent opinion, their elaboration is not so much an index of intelligence as are the subcortical white fibers beneath, which make possible the innumerable conditioned reflexes that transform the cerebrum into an effective organ of integration. “The attempt to make a great deal of this feature yields but little comfort,” as Stiles says, “since the sheep and cow have deeply furrowed cortical surfaces, while some apes, with much more intelligence, have nearly smooth brains.”
Nevertheless the fissures do serve as boundary lines for certain regions into which the cerebrum may be parceled out topographically for convenience in localization and description. Such, for example, are the frontal, parietal, occipital, and temporal lobes, corresponding roughly to the cranial bones that cover them. The frontal and parietal lobes are marked off from one another by the central sulcus (Fissure of Rolando) while these two lobes are separated from the temporal by the large lateral cerebral fissure (Fissure of Sylvius).
A century ago two anatomists, Goll and Spurzheim, attempted to associate certain human “faculties” with definite areas in the cortex. What began on their part as a legitimate scientific inquiry and experiment, soon developed into the quagmire of phrenology. This pseudo-science gained great temporary vogue in the hands of clever charlatans who knew very little about the brain. Trading upon the ignorance of the general populace, wandering “Professors of Phrenology,” by skillful guesswork and owlish digitation of “bumps” on the cranium, covered up their lack of real knowledge of cerebral localization and were usually able to satisfy their credulous patrons as to the great hidden capacities of any particular brain. Thus “mute inglorious Miltons” and unrecognized rural Napoleons were discovered everywhere at fifty cents a head! Today the discredited phrenologist properly belongs with the witch doctor, palm reader, clairvoyant, fortune teller, and astrologer, although it must be admitted that many of his blood relatives, who would seek a short cut to the truth that may be gained only by slow and patient travel over a long road, are still abroad and active in many guises.
There is, however, a discoverable localization of different functional centers in the cortex. This fact has been abundantly demonstrated beyond all doubt, both clinically and experimentally, as well as pathologically, by the examination of brain lesions and defects in autopsies following various types of local paralysis.
One of the earliest proofs of the cortical localization of a specific motor center was furnished during the Franco-Prussian War in 1870, when two surgeons, Fritsch and Hitzig, operating upon a wounded soldier, accidentally demonstrated a definitely located muscular response when a certain area of the exposed cerebrum was subjected to slight galvanic stimulation.
The knowledge of brain localization that has been pieced together in the last half century is of the greatest importance. By observing the part of the body that is suffering from paralysis the surgeon is able to know without preoperative exploration exactly the spot in the cortex where a blood clot, tumor, or lesion is situated.
The location of some of the cortical centers is shown in Fig. 640. It will be noted that the centers there shown do not represent mental or psychological characteristics, but instead centers of correlation, association, or projection, in direct relation to external sense organs and effectors of various kinds. Even a process as obscure as thinking is not different in kind from other functions of the brain and may be referred in the last analysis to the operation of reflex arcs with corresponding relationships between neurons.
Although “specific mental acts or faculties are not resident in particular cortical areas,” according to Herrick, yet physiological centers, definitely assigned to particular tasks, similar to the arrangement of buildings in accordance with zoning laws, have been mapped for the entire surface of the cerebrum.
The phenomena included under the general term of aphasia, which may be manifested in various guises such as the loss of the power of speech (aphernia) ; the loss of ability to understand spoken words (auditory aphasia) ; the loss of ability to read printed or written language that was formerly understandable (alexia) ; or the loss of ability to write (agraphia), are all found to be associated with lesions or defects in definite regions of the cerebral cortex. There even seem to be different centers for different languages, since a bilingualist may lose by brain lesion the ability to speak one of two languages in which he was formerly proficient.
Distributed along the anterior edge of the central sulcus, on the side of the brain opposite to the region of the body involved is the motor cortex in which are the principal motor centers for specific regional muscles. The parts of the body are represented in inverted order, the area for the toes being the most dorsal, while those for the lace are the most ventral (Fig. 641).
The left frontal lobe of the cerebrum is the location of the speech center (in right-handed persons); the occipital lobe, of the visual center; and the temporal lobe, of the auditory center. All of these lobes are enlarged in connection with the power of speech with which sight and hearing are intimately associated in the case of man. Immediately posterior to the central sulcus in the parietal lobe, is the somatesthetic (somesthetic) area, the center for sensation of touch, pressure, pain, and temperature as well as proprioceptive sensibility from the muscles, tendons, and joints (Fig. 640). As in the case of the motor cortex the parts of the opposite side of the body are represented in inverted order with the result that the motor and sensory areas for any individual part are at the same level on either side of the central sulcus.
The brain not only receives impressions through the sense organs but it also records them by means of the mechanism furnished by the cortex for storing impressions that may be later revived by memory. As has been aptly said: “When we wake in the morning the sheet of gray cortex on the brain becomes the screen on which is lit up the cinema of the outside world” (Keith). It has been estimated that the cortical fibers of a single human brain if placed end to end would reach a distance equal to that from New York to Paris, a fanciful and incomprehensible guess that probably falls short of the truth.
The billions of cells composing the gray cortex of a single human brain all together represent a mass of less than a cubic inch of material that weighs only about thirteen grams, or approximately one five-thousandth of the total body weight. The value of this precious cubic inch of tissue is summed up by Parker in the following words: “When it is recalled that the 92,200,000,000 cells in the human cerebral cortex are the nervous elements of this organ and that they collectively constitute rather less than a cubic inch of protoplasm, it seems almost incredible that they should serve us as they do. They are the materials whose activities represent all human mental states, sensations, memories, volitions, emotions, affections, the highest flights of poetry, the most profound thoughts of philosophy, the most far-reaching theories of science, and, when their action goes astray, the ravings of insanity. It is this small amount of protoplasm in each of us that our whole educational system is concerned with training and that serves us through a lifetime in the growth of personality.”
Craniospinal Nervous Pathways
Of the different bundles of fibers connecting brain and spinal cord, four groups were mentioned in the discussion of the cord. Three of these were the dorsal columns (fasciculi gracilis and cuneatus), the spinothalamic tracts, and the spinocerebellar tracts, all of which are made up of ascending fibers; the fourth was the descending corticospinal group. We are now in a position to consider some of the cranial portions of these pathways.
Fasciculi gracilis and cuneatus
As mentioned during the discussion of the spinal cord, sensory neurons with their cell bodies in the dorsal ganglia send their neurites into the dorsal funiculus. Many of these neurites continue forward as the fasciculus gracilis, in the case of sensory neurons from posterior parts of the body, or the fasciculus cuneatus, for anterior sensory neurons. These bundles extend into the medulla as the funiculi gracilis and cuneatus, respectively. Here the neurites terminate, synapsing with the short dendrites of relaying neurons, or sensory neurons of the second order, which carry the impulses to more anterior parts of the brain (Fig. 642).
The neurons of the second order for the fasciculus gracilis have their cell bodies in the nucleus gracilis which lies near the region where the roof plate is broadening to form the thin velum (Fig. 633). The nucleus cuneatus, for the relaying neurons of the cuneate tract, is just lateral to the gracile nucleus. Most of the neurites of these relaying neurons swing in an arc ventrally to decussate (cross to the other side of the medulla), beneath the central canal and posterior part of the fourth ventricle, thus forming what is known as the sensory decussation. As soon as they cross the mid-line they turn sharply to run anteriorly, as the median lemniscus which lies near the mid-line.
In the thalamus the fibers of the second order terminate and synapse with sensory neurons of the third order which have their cell bodies in one of the thalamic nuclei. The neurites of these third neurons go to the somatesthetic area of the cerebral cortex.
These tracts form pathways for sensations of touch and pressure as well as for proprioceptive impulses from the muscles, joints, and tendons.
In the case of the spinothalamic tracts the sensory neurons (neurons I) synapse with neurons II within the dorsal gray matter of the cord. The neurites of these neurons of the second order cross, through the ventral commissure, into the spinothalamic tracts of the other side of the cord (Fig. 643). The fibers of each of these tracts reach the thalamus, where they synapse with the neurons of the third order which have their cell bodies in one of the thalamic nuclei. From this nucleus, the neurites extend to the somatesthetic area of the cerebral cortex.
It will be recalled that physiologically the two tracts of this group are not the same. The ventral spinothalamic tract is a pathway for sensations of touch and pressure, the lateral spinothalamic for pain and temperature. Also the neurites of the primary sensory neurons associated with each of these tracts take somewhat different courses. The neurites carrying touch and pressure impulses run in the dorsal columns before sending collaterals in the dorsal gray horn. The neurites carrying pain and temperature impulses go directly into the gray matter at the level of entrance into the cord.
The spinocerebellar tracts are important proprioceptive pathways to the cerebellum, the chief center of correlation of these impulses as well as of coordination of muscular activity. In both of these tracts the primary sensory neurons are similar to those described for the fasciculi gracilis and cuneatus. Collaterals, leaving these neurons along their course, enter the dorsal gray matter to synapse with the neurons of the second order.
The cell bodies of the dorsal spinocerebellar tract are located in the nucleus dorsalis (Clarke’s column) from which neurites run into the dorsal tract of the same side of the cord. These fibers run forward through the medulla and enter the cerebellum by way of the restiform body (Fig. 644).
The ventral spinocerebellar tracts are composed of neurites whose cell bodies are in the dorsal horns and adjacent portions of the gray matter. Some fibers enter the nearby tract of the same side of the cord; others cross over, through the ventral commissure, to run forward through the ventral tract of the opposite side. Each ventral spinocerebellar tract passes through the medulla and deeper part of the pons to enter the anterior cerebellar peduncle, through which it swings back into the anterior part of the cerebellum (Fig. 645).
Numerous motor pathways eventually leave the spinal cord through the primary motor neuron or “final common path,” as mentioned previously. Many groups of fibers exert involuntary control over the muscles, while several which descend from the cerebral cortex place the skeletal muscles under voluntary control. The most important voluntary tracts entering the spinal cord are the corticospinals which originate in giant pyramidal cells of the motor cortex, immediately anterior to the central sulcus.
The neurites of these pyramidal cells pass successively through the white matter of the cerebral hemisphere, the crura cerebri and the deeper part of the pons to enter the pyramids of the medulla oblongata (Fig. 646). In the pyramidal decussation, at the posterior end of the medulla, most of these fibers cross over to become the lateral pyramidal, or lateral corticospinal, tract which continues the entire length of the cord. At every segment some of these fibers enter the ventral horn of the gray matter to synapse with primary motor neurons. In man, about one-fourth of the fibers do not decussate but continue posteriorly as the ventral pyramidal tract, which ordinarily does not extend below the mid-thoracic region. When these direct fibers leave the tract, a few at a time, they cross to the other side of the cord and enter the ventral gray horn where they synapse with the primary motor neurons. Thus while in one case the fibers cross in the medulla, in the other they cross near their termination in the cord; but all of them cross. Consequently all of the skeletal muscles of one side of the body are under the voluntary control of the motor cortex of the opposite side of the brain.
The Comparative Anatomy of the Brain
The essential features of the comparative anatomy of the vertebrate brain may be passed in review by an examination of diagrams of the brain of various vertebrate types (Figs. 647-652) which may be compared also with the diagram of a vertebrate brain based principally upon that of a cyclostome (Fig. 630).
The brain of the lamprey eel, Petromyzon, is without marked flexures and quite primitive in plan. Connected with the relatively large medulla are six pairs of cranial nerves (V-X, inclusive), there being only ten pairs in these animals, as in fishes and amphibians. The roof of the fourth ventricle has an extensive vascular network.
The metencephalon of Petromyzon includes a rudimentary lip-like cerebellum but no pons, while both are lacking in Myxine. The roof of the mesencephalon has but one pair of swellings, the optic lobes. As the wall of this region remains relatively thin there are large optic ventricles which are not separated from the aqueduct.
The infundibular region and the thalamus proper are small. The epithalamus includes a pair of habenular ganglia connected by a commissure as in all vertebrates. There are two outgrowths from the habenular region: a well developed epiphysis (pineal organ), anterior to which is a smaller parietal body. These two organs, arising from a common origin, were probably originally paired, since the pineal body is in intimate relation with the right habenular ganglion, and the parietal body with the left.
The presence in certain fossil fishes, for example Titanichthys, of a pair of foramina located side by side in the skull directly above this region in the brain, also seems to point to the originally paired relation of these organs. Apparently the tandem-like position of the pineal and parietal bodies in cyclostomes is the result of a secondary displacement of the former side-by-side arrangement.
The histological structure of the epiphysis, or pineal body, as well as its access to light through a foramen in the dorsal region of the skull, seems to indicate that it is, in cyclostomes at least, a photoreceptive organ by means of which light and darkness are distinguished.
The rather small telencephalon, olfactory in function, includes paired olfactory bulbs, olfactory lobes, and weakly differentiated corpora striata. An anterior commissure, found in all vertebrates, connects the two sides of the brain in the region of the lamina terminalis.
In elasmobranchs the bulging corpora striata, which even grow into the lamina terminalis, and the dominant olfactory lobes constitute the bulk of the telencephalon (Figs. 647 and 653). From each olfactory lobe there extends forward a conspicuous olfactory stalk, terminating in an olfactory bulb which may be quite large. The velum interpositum, the thin roof of the third ventricle, develops an invaginated transverse fold, or velum transversum, which marks the posterior limit of the dorsal part of the telencephalon.
In the diencephalon the epiphysis is stalked, reaching as far as the cartilaginous roof of the cranium in many cases. The parietal organ disappears after temporary appearance during embryonic development. The infundibulum develops a pair of elongate swellings, the inferior lobes, between which the hypophysis is attached. Posterior to the inferior lobes and dorsal to the neurohypophysis proper, is a thin-walled vascular sac, or saccus vasculosus, lined with a sensory epithelium of problematical significance. This sac is not found in cyclostomes or tetrapods.
The well developed optic lobes may be partially covered by a large cerebellum. As would be expected, the cerebellum is much larger in the active dogfishes and sharks than in the sluggish skates and rays.
In the antero-lateral part of the medulla, connecting it with the cerebellum, are prominent restiform bodies which stand out like a pair of ears on either side of the cerebellum. In elasmobranchs, as in embryonic stages of higher vertebrates, the differentiation of the medulla into four longitudinal areas is clearly visible. These four columns represent the same four nervous components described for the cord. On either side of the thin ependymal velum which forms the roof of the fourth ventricle are the somatic sensory columns ventral to which are the visceral sensory columns, then the visceral motor columns, and finally the somatic motor columns which lie on either side of the median plane in the floor of the ventricle (Fig. 654).
The electric Torpedo has two electric lobes (Fig. 647) that project into the fourth ventricle. These structures are hypertrophied nuclei of the vagus (X) nerve which supplies the electric organ of these animals.
In bony fishes the brains are small and show great variations. The roof of the telencephalon is thin and non-nervous (Fig. 648). Convex corpora striata occupy the floor of this region. The shortened epiphysis is on the downward road of degeneration. The optic lobes are large, especially in teleosts. The cerebellum, small in dipnoans and ganoids, is large in most teleosts, especially the most active ones.
In amphibians (Fig. 649), the large elongated olfactory lobes, which are continuous with the distinctly separated cerebral lobes, lie closely side by side and are joined medially together.
The corpora striata project upward from the telencephalic floor only slightly, allowing for fairly large lateral ventricles and invaginated anterior choroid plexuses inside the cerebral lobes. The latter are without an external layer of gray matter, although scattered neurons begin to appear in the thickening pallial wall.
The diencephalon is uncrowded, and visible from above, without inferior lobes or saccus vasculosus. In adult anurans the epiphysis is represented by a small median vesicle, the pineal gland, close under the dorsal wall of the cranium, which by its development is shown to be the bulbous tip of the vanished stalk of the epiphysis. Skulls of stegocephals have a dorsal foramen, which shows the presence in those primitive amphibians of either a pineal or a parietal eye in this region. The parietal body is absent in modern amphibians.
The optic lobes have been so spread apart as to occupy a more lateral position, and the cerebellum, reduced to a transverse lip in most forms, is quite rudimentary in the caecilians as well as in some urodeles.
The reptilian brain (Fig. 650) shows an advance in the telencephalic region, since a gray cortex, although not pronounced, is definitely laid down, and the commissures between the cerebral lobes are somewhat more developed than in the amphibian brain.
The corpora striata are so large that only small lateral ventricles remain. In most reptiles the olfactory lobes are hardly distinguishable from the neopallial part of the telencephalon, but in those lizards and alligators with prominent projecting snouts the olfactory lobes are extended into stalks and bulbs, as in elasmobranchs.
The diencephalic region of the brain of reptiles is of particular interest. The thalami are large, while the hypophysis attached to the infundibulum is definitely differentiated into an anterior and a posterior part, the former enveloping the latter. On the thin dorsal wall of the third ventricle there are, at least embryonically, not only pineal and parietal outgrowths, but also a third evagination, the paraphysis, which is so far anterior in position that it actually belongs to the telencephalon. The paraphysis usually undergoes degeneration in adult life and its function is still unknown.
Except in crocodiles and alligators, the epiphysis is always present as a glandular pineal structure. The parietal organ is also always present in close association with the same habenular centers as those of the pineal body. Reaching its highest development in Sphenodon, in which it extends as far as a transparent window on the roof of the skull, it is unmistakably a third median eye, equipped with a retina and a lens.
In snakes distinct corpora quadrigemina occur in the dorsal part of the mesencephalon. It has been shown, however, that other reptiles and possibly amphibians have small auditory centers, corresponding to posterior colliculi.
These auditory centers do not show as a pair of elevations until the optic centers become reduced to anterior colliculi occupying only the anterior part of the roof of the mesencephalon. The thickening of the mesencephalic wall reduces the cavity of this region to a slender canal, the aqueduct.
The cerebellum of reptiles is usually small, although fairly well developed in swimming forms. Eight pairs of cranial nerves (V-XII inclusive) connect with the medulla as in birds and mammals, whereas only six pairs (V-X inclusive) occur here in anamniotes.
The brain of birds (Fig. 651) is more of an “eye brain” than a “nose brain,” thus showing an advance over its forerunners. The cerebral cortex, however, is less well developed than in reptiles, with the corpora striata more in evidence. The lateral ventricles, due to the increased thickness of the corpora striata, are reduced to very restricted spaces.
The olfactory lobes were fairly prominent in the tooth-bearing cretaceous birds, but are small and degenerate in their modern representatives. No trace of a parietal organ appears in birds, and since the degenerate pineal body is buried between the encroaching cerebrum and cerebellum, the entire brain gives the impression of compact crowding and centralization, characteristic of the bird’s structure generally. This is due not only to the fact that the greatly enlarged optic lobes are crowded over laterally in position, but also to a considerable backward growth of the cerebrum which tends to bury the diencephalon and the mesencephalon from dorsal view.
The cerebellum, consisting of a well-defined median vermis as well as two lateral lobes, is very large, as might be expected in these extremely active animals.
Tertiary mammals, as methods of exploring the cranial cavity of fossil skulls reveal, had a reptilian type of brain. In modern mammals also, the brain (Fig. 652) is more like that of reptiles than that of birds, since the outstanding size of the cerebrum is due to the development of the cortex rather than to an enlargement of the corpora striata, as is the case among birds.
The commissural systems between the cerebral lobes are better developed than in other vertebrates, particularly by the elaboration of the large corpus callosum, although this is small in the monotremes and marsupials.
While still prominent in the monotremes, marsupials, and other lower mammals, the archipallial olfactory part of the brain becomes reduced among the higher mammals, until in man it is very small, and in seals and whales almost entirely lacking.
In the diencephalon the thalami are large. The epiphysis, now connected with endocrine activity and having lost its eyelike structure, is reduced to an organ degenerate in size, although indispensable in function. It is usually covered over by the hemispheres and is relatively large in ungulates and rodents but missing in armadillos and other edentates.
The two optic lobes of lower forms become changed in mammals into four corpora quadrigemina, which are relatively smaller than is this region of the mesencephalon in any other vertebrate. The corpora quadrigemina are quite covered by the overgrowth of the massive cerebrum and the cerebellum which have so enlarged that they meet dorsally.
In addition to enlarged vermis and lateral lobes, flocculi of considerable size are present (Fig. 655). Thus the correlation tissue of the cerebellum is notably increased.
A further characteristic of the mammalian brain is a definite band of fibers encircling the brain stem, known as the pons, in the metencephalic region. The medulla of mammals is comparatively short, and appears to be drawn under the prominent cerebellum.
The Control of the Body by the Brain
Animal activities having their physical basis in the brain are of three general categories, namely, (1) innate stereotyped functions, which are inherited, and are ordinarily blanketed together under the term instinct; (2) habits, which are not inherited but are patterns of conduct acquired by repetition until they become more or less automatic; and (3) variable modifiable actions, which are marks of intelligence that are perfected by the process of learning.
Innate stereotyped functions, it may be said, do not lend themselves to individual improvement. It is futile, for example, to try to teach a grasshopper either to jump or to dance. It jumps already, having been born an instinctive jumper, and it can never learn to dance, since the “variable, modifiable” capacity is practically wanting in its make-up.
With an increase of cerebral function the instinctive automatic reflexes take more and more to the background, and therein is the great distinction between “lower animals,” that are largely at the mercy of their environment and heredity, and the “higher animals,” which to an increasing degree have risen above environing conditions and their hereditary handicaps, and have become more and more “captains of their souls.” One of the most prized possessions of mankind is the “capacity for individuality,” yet even what passes for “free will” has its basis in the neurons and reflexes built up in the brain. In the last analysis the brain must be regarded as the mechanism through which consciousness, memory, imagination, and will are effected.
The control which the brain, and particularly the cerebral cortex, exercises over the body is increasingly greater as one passes from fishes to mammals. A “spinal frog,” in which the brain has been destroyed but the spinal cord left intact, continues to perform many of its functions in an apparently normal fashion. A hen with its head cut off continues to flop about for some time, but not so a mammal that has been guillotined. In man a comparatively slight interference with even a minute portion of the cortex may result in sudden fatal apoplexy.
An important function of the brain and the cord that should be mentioned is their activity as inhibitors of many of the myriad reflexes called forth continuously by an insistent environment. Without such automatic inhibitions man would be worn out by continuous responses to a great variety of stimuli to which he is constantly exposed. Relief from incessant activity is gained not only by the inhibitory action of the nervous system, but also by periods of unconsciousness during sleep. It has been estimated that a person seventy years of age has lost consciousness over 25,000 times in sleep.