FIG. 25. — Schematic drawing to show neural elements involved in a simple reflex arc. 1, afferent peripheral neurone; 2, central or connecting cell; 3, motor neurone; MS, striped muscle; SS, sensory surface (skin).
as to the kind of connection existing among neurones. It seems safe to say that it is usually one of contact like that shown in Fig. 26 — the end processes of an axorie or its collateral come into more or less intimate contact with the bushy dendrites or cell body of another neurone. The place where functional con- tact occurs is called a "synapse." Many physiologists believe that the synapse is the most important part of the reflex arc. If the relation is one of contact there must be a surface of separation there, and this, it seems, in some way affects conduction. An impulse takes longer to traverse a reflex arc which always con- tains one or more synapses than to pass an equal distance over a NEURO-PHYSIOLOGICAL BASIS OF ACTION 119 nerve trunk. Furthermore, an impulse may travel in either direc- tion along a nerve fiber, but it can travel in only one direction (forward) through the reflex arc, i.e., from the axones to the dendrites. The idea has gained ground that the synapse can offer varying resistance to the passage of a neural impulse. It is as- sumed that an impulse coming from a sense organ might very well pass out to the muscles over either one of two neurones, but the mo- mentary resistance at the one sy- napse may be so high that the im- pulse can pass out only over the other. It can readily be seen that this hypothesis may be of help in explaining habit, failure to obtain the predicted response, sleep, etc. The All-or-None Law.— If the all-or-none law is established the conception of the synapse will have to be modified. The all-or-none law states that if a nerve fiber is stimulated it is stim- ulated maximally in each and in all of its parts. Hence grading of neural activity by the action of the synapse is not possible, for if the propagated disturbance (neural impulse) gets by the sy- napse at all, where it may suffer a decrement, it will become max- imal again after a short distance. This can be shown quite clearly in the case where a nerve trunk is being stimulated electrically. The resulting action current is taken as the measure of the impulse and is registered by means of the deflections of the string of a galvanometer. If we partially anaesthetize the fibers in a stretch of a nerve trunk and measure the amount of deflection in the area so narcotized, the magnitude of the deflection is de- creased; but if we measure the propagated disturbance (impulse) L .
FIG. 26. — Termination of axones around cell bodies. A, B, and C, three cells with axones terminating upon them (from reception nucleus of cochlear nerve of rabbit); a, a, a, fibers of the cochlear nerve which break up into terminal arborizations upon the cells; 6, c, terminal rings. The points of contact are called a synapse; the points of contact occur more frequently at the dendrites than at the cell body. (Bailey's Text-Book of Histology.) Courtesy Wm. Wood & Co.
120 PSYCHOLOGY 120 PSYCHOLOGY shortly after it passes out from this region ^here it underwent a decrement we shall find that the deflection of the galvanometer string has returned to its normal magnitude. Grading of a muscu- lar response then must be a function of the number of muscular fibers thrown into action, an'd this is a function, in turn, of the number of axones bearing impulses to the muscle. Slight stimula- tion of a nerve trunk will produce a small contraction of the muscle because with a slight stimulus only a few nerve fibers are aroused. As soon as all of the fibers are bearing impulses no in- crease in the intensity of the stimulating current will produce a larger contraction. Our common experience teaches us that there is a rough proportionality between magnitude of stimulus and magnitude of response. But this grading is probably not a func- tion of the synapse nor does it take place in the central nervous system.1 The all-or-none law must still be looked upon as a re- search problem. If the various implications of the law are con- firmed it will probably profoundly modify many of the present conceptions of neuro-physiology.
Some of the Facts Known About Neural Action. — It has been shown that the speed of the neural impulse in the motor nerve of man is about 125 meters per second. Apparently the velocity may be altered in various ways. Variations in temperature alter it most markedly. If one starts with a low temperature and tests the velocity it is found that for each 10° C. rise in temperature the velocity is doubled until the physiological limit is reached. Cooling a section of the nerve beyond a certain point will block the nervous impulse. Anaesthetics and narcotics may also be ap- plied locally to the nerve, both decreasing its irritability and conductivity, or suspending them entirely. Conductivity and irritability may also be suspended by depriving the nerve of oxygen. With the restoration of oxygen these functions are re- stored. The questions as to whether activity can fatigue the nerve fiber as it is known to fatigue the cell body, and whether 1 We shall see later, however, that impulses from the cerebral structures^ may inhibit action in lower motor neurones. This, however, is a function of the refractory phase of the neurone and of the time relations of the impulses impinging upon it, NEURO-PHYSIOLOGICAL BASIS OF ACTION 121 nerve fibers show chemical change after or during activity, have not been answered with complete certainty.2 Certainly the nerve fiber is fatigued with the very greatest difficulty in ordinary lab- oratory experiments where the electrical current is used to excite it. That both the resting and active fibers show metabolic change is becoming generally admitted. Recent work tends to show that the resting fiber gives off CO2, that it eliminates it faster during functional activity and that the elimination is as great per unit of weight as that of the cell body. There can thus be little doubt that since oxygen is needed also by the fiber to maintain its func- tional properties, and since it gives off C02, functional activity in the fiber is connected with a chemical reaction of some kind.
Nature of the Neural Impulse. — The tendency is growing to regard the neural impulse as the rapid passage of a wave of chem- ical decomposition. If a hair on the skin is touched it is assumed that the structure and composition of the surface film (surface films must exist between two structures which are in contact) of the axone ending around the hair is altered. ' ' The state of elec- trical surface polarization is thus changed; and the bioelectric circuit arising between altered and adjoining unaltered regions completes the activation" (Lillie). These local currents extend for only a few centimeters, but at the point where they end there is the condition at hand for starting a new disturbance of film (between the portion of the nerve which has just been active and the resting portion which joins it) and the process is thus re- peated the whole length of the conducting arc. The speed of prop- agation of the wave or impulse is thus slow notwithstanding the 2 The question of the fatigue in nerve cells, while generally admitted, has not been very well worked out. There seems to be agreement that certain histological changes can be noticed after a cell has been made to function severely. There is possibly an increase in the size of the cell, a diminution of the chromatic (Nissl) substance, and possibly even a displacement of the nucleus (Hodge). In prolonged activity the chromatin may completely disappear. Presumably in the cell body as in the active muscle lactic acid and CO2 are formed. Certainly oxygen is necessary for the proper function- ing of the cell. Very little can be said with certainty about the role the cell body plays during the conduction of the neural impulse. It is certainly best to look upon the cell as a nutritive center of the neurone as a whole.
122 PSYCHOLOGY 122 PSYCHOLOGY essential electric nature of the phenomena. Lillie gives the process as follows: "The rapid passage of waves of chemical de- composition (probably oxidative in nature and involving some structural change) over the surface of the reacting element, fol- lowed immediately by a reverse change which restores the original or resting condition, is what appears to take place in a nerve or other living structure during conduction. Associated with the chemical process is a local electrical circuit by whose electrolytic action the chemical change is apparently determined. " THE CEREBRO-SPINAL SYSTEM (SYSTEMA NERVORUM CENTRALE).3 Introduction. — The brain and spinal cord with their various peripheral connections may be looked upon as a unitary aggre- gation of simple and complex reflex conduction systems such as we have just considered. The brain and cord connected on the one hand with the sense organs and on the other hand with the muscles and glands afford a multiple connection system between the various receptors and the various effectors. No matter how minute the sense organ structure is which is stimulated, the im- pulse arising there can travel to the central system and produce a response of the whole organism which is entirely out of pro- portion to the actual energy applied at the sense organ. In other words, a stimulus applied anywhere on the body produces not only a local segmental reflex action, but it changes the system of tensions and secretions probably in every part of the body.
To understand how the nervous system is put together we must first spend some time upon the gross or macroscopic features of the brain and spinal cord, coming back finally to the discussion of internal architecture and inter-relations among the neurones themselves. Once the gross structures are found they will serve as landmarks in the description of the various pathways in the brain and cord.
The brain and cord together are known as the central nervous system (systema nervorum centrale), but as we have pointed out the central nervous system is connected on the one hand with a • For the sake of ease in referring to the cuts where the scientific or Latin name of the part is given in the singular, we follow the same usage in the -body of the text.
NEURO-PHYSIOLOGICAL BASIS OF ACTION 123 SUPERIOR CERVICAL SYM- PATHETIC GANGLfON KIDDLE CERVICAL SYMPATHETIC GANGLION I THORACIC XERVBpANGLIATED CORD \ \ GANGLION - -I SACRAL NERVE \ COCCYG£AL NERVE FILUM TERMTXALE FIG. 27. — The human central nervous system from the ventral side illustrating its con- nections with the cerebro-spinal nerves and with the sympathetic nervous system (in black, on right side only). The various subdivisions of the cord are shown. (Herrick's Introduc- tion to Neurology.)
124 PSYCHOLOGY 124 PSYCHOLOGY sense organ by means of the afferent peripheral cerebro-spinal neurones, and on the other hand with the muscles by means of the efferent peripheral cerebro-spinal neurones. These latter divi- sions are often spoken of as the peripheral nervous system (sys- tema nervorum periphericum). Usually the sympathetic or auto- nomic system (sy sterna nervorum sympathicum) is included as a part of the peripheral nervous system. We shall leave the sym- pathetic out of account at present, giving it a separate treat- ment on page 154.
'Gross Features of the Spinal Cord (Medulla Spinalis). — The spinal cord is shown together with the brain in Fig. 27. The cord is about eighteen inches long and extends from the first cervical vertebra (more accurately from the foramen magnum of the occipital bone) to the lower part of the body of the first lumbar vertebra. Its upper portion is continuous with the medulla oblongata, which is the lowest part of the brain. The lower portion of the cord tapers conically and ends in a slender filament, the filum teminale. The cord is invested with three membranes. These are shown in Fig. 28: (1) the dura mater (dura mater spinalis) is a tough protective membrane which forms a lining for the bony cavity; (2) a thin intermediate mem- brane, the arachnoid (arachnoidea spinalis); and (3) finally a highly vascular membrane which closely invests the neural structures, the pia (pia mater spinalis).
The cord is almost cylindrical in shape with two enlargements, a cervical (intumescentiacervicalis) and a lumbar (intumescentia lumbalis) (Fig. 27). The spinal nerve roots leave from regular segments in the cord (Fig. 36). There are thirty-one such seg- ments corresponding to the thirty-one spinal nerves. It will be noted that the cord in this respect is symmetrical in that there are thirty-one nerves on each side. The cord is divided into white and gray matter. The outer portion of the cord is com- posed of white matter (substantia alba) while the central H-shaped portion is composed of gray matter (substantia grisea). The white matter is made up largely of myelinated axones coming from the spinal ganglia (to be described on page 138) and the myelinated axones growing out of the cells lying in the gray NEURO-PHYSIOLOGICAL BASIS OF ACTION 125 matter. The gray matter is made up largely of nerve cells with their dendrites and of the unmyelinated end brushes of axones ending around these cell bodies. In Fig. 29 attention is called to the dorsal median septum (sulcus medianus posterior) and to the ventral median fissure (fissura mediana anterior). These structures serve to orient one immediately with respect to the ventral and dorsal aspects of the cord. The ventral aspect of the FIG. 28. — Th£ membranes of the spinal cord. 1, the dura mater (dura mater spinalis); 2, arachnoid (arachnoidea spinalis); 3, pia (pia mater spinalis). In the drawing the arachnoid is too strongly emphasized. Only rarely can one separate this membrane in a cord dissection. (Modified from Toldt.)
cord faces the ventral cavity of the body. Since the cord is sym- metrical usually, only one-half of it is described. We can divide the white matter of each half of the cord as follows (Fig. 29): (1) a ventral funiculus (funiculus anterior, 6), (2) a dorsal funiculus (funiculus posterior, 11), and (3) a lateral funiculus (funiculus lateralis, 9 ). The gray matter can similarly be divided into a (1) dorsal column — dorsal horn of gray matter — (columna posterior, 12), (2) ventral column — ventral horn of gray matter — (columna anterior, 7), and (3) lateral column — lateral horn of gray matter — (columna lateralis, 8).
PSYCHOLOGY PSYCHOLOGY Extending through the entire length *f the cord there is a small central canal (canalis centralis) to be found in the sub- stance of the gray commissure. It is the remains of the primitive ectodermal canal. The canal is the counterpart of the several ventricles in the brain, at least so far as origin is concerned. The canal and the fourth ventricle communicate at the cala- mus scriptorius.
Gross Features of the Brain (En- cephalon). — The ventral aspect or base of the brain is shown in Figs. 30 and 31. The brain lies almost horizontally in the cranial cavity. We shall give a short description of the various features of the brain, beginning with the first. or lowest structure. We shall try to describe to- gether the various structures which be- long together, but occasionally it has seemed better for the sake of clearness not to adhere too closely to the neurol- FiG.29.— Thesubdiyisionsof Ogist's divisions.
The Medulla Oblongata, Pons, Cere- "™ bellum and its Peduncles. — At the upper level of the cord and continuous with it S>} is to be found the medulla oblongata (Fig. 30, 1). On the ventral aspect of the homVo?tS1ayCmatter medulla we find the pyramids (pyramis) (Fig. 30, 26). Just lateral to the pyramatter (columna lateralis).
3). On the dorsal aspect of the medulla (Fig. 32) are to be found the inferior cerebellar peduncles (corpus restiforme) (Fig. 32, 21), a band of fibers connecting the cord and medulla with the cerebellum. On the posterior aspect of the medulla one also finds two slight swellings, the tuberculum cuneatum (Fig. 32, 23) and the clava (Fig. 32, 4). At the upper border of the medulla and continuous with it is to be found the pons (Varoli) (Fig. 30, 8). The pons is really a great band of transverse axones coursing about the ventral aspect of the brain stem. Its fibers NEURO-PHYSIOLOGICAL BASIS OF ACTION 127 connect the two hemispheres of the cerebellum. The fibers are called commissural.4 In addition to the superficial transverse axones, there are axones olmany neurones to be found at this 25 26 3 27 FIG. 30. — View of the base of the brain. 1, spinal cord (medulla spinalis); 2, decus- sation of pyramids (decussatio pyramidum); 3, spinal accessory nerve (n. accessorius); 4, cerebellum; 5, choroid plexus of fourth ventricle (plexus chorioideus ventriculi quarti); 6, flocculus; 7, abducens nerve (n. abducens); 8, pons (Varoli); 9, oculomotor nerve (n. ocu- lomotorius); 10, temporal pole (polus temporalis); 11, fissure of Sylvius (fissura cerebri lateralis (Sylvii)); 12, hypophysis; 13, olfactory tract (tractus olf actorius); 14, frontal pole (polus frontalis); 15, olfactory bulb (bulbus olfactorius); 16, optic nerve (n. opticus); 17, optic tract (tractus opticus); 18, ganglion, root and branches of the trigeminal nerve (n. trigeminus); 19, trochlear or fourth nerve (n. trochlearis); 20, intermediate nerve (n. intermedius); 22, facial nerve (n. facialis); 23, auditory nerve (n. acusticus); 24, vagus and glossopharyngeal nerves (n. vagus, n. glossopharyngeus); 25, hypoglossal nerve (n. hypoglossus); 26, pyramids (pyramis); 27, first cervical spinal root.
PSYCHOLOGY FIG. 31. — Ventral aspect of medulla and pons. 1. pyramidal decussation (decussatio pyramidum); 2, spinal accessory nerve (n. acces- •orius): 3, olive (oliva); 4, vagus and glosso-pha- ryngeal nerves (n. vagus, n. glossopharyngeus); 5, middle cerebellar peduncle (bracnium pontis); 6. abducens nerve (n.abducens); 7, pons (Varoli); 8, oculomotor nerve (n. oculomotorius); 9, optic tract (tractus ppticus); 10, mammillary body (corpus mammillare); 12, olfactory fibers (stria olfactoria lateralis); 13, trigonum olfactorium; 14, optic chiasm (chiasma opticum); 15, olfac- tory bulb (bulbus olf actorius); 16, olfactory tract (tractus olf actorius); 17, optic nerve (n. opticus); 18, hypophysis; 19, anterior per- forated substance (substantiaperforata anterior); 20, uncus of hippocampal gyrus (uncus gyri hippocampi)); 21, cerebral peduncle (pedunculus cerebri); 22, trigeminal nerve (n. trigeminus); 23, facial nerve (n. facialis); 24, intermediate nerve (n. mtermedius); 25, acoustic nerve (n. acusticus); 26, hypoglossal nerve (n. hypo- g 088U8); 27, lateral funiculus (funiculus later- alls); 28, motor root of first cervical nerve.
level wfiich form connecting pathways between higher and lower levels of the central nervous system. The trans- verse fibers of the pons form the middle, cerebellar ped- uncle (brachium pontis) (Fig. 32, 20). In addition to the axone system in the pons, there are a number of gray cellular masses which serve as nuclei of reception for the sensory roots of cranial affer- ent nerves which enter in or near the pons and others serv- ing as nuclei of origin of the motor nerves.4 The cerebellum or hind brain (Fig. 30, 4) lies dor- sal to the pons and me- dulla and overhangs the lat- ter. It in turn is overhung by the occipital lobes of the cerebral hemisphere (Fig. 35). The cerebellum is a fairly large structure weigh- ing about 140 grams. "We have already spoken of two of its peduncles, the middle or pons, and the inferior (the medulla and cord connection). It is connected with the higher brain centers by the superior 1 A convenient way to describe the axone system is to use the terms ( 1 ) projection neurones, (2) commissural neurones, and (3) association neu- rones, meaning respectively ( 1 ) neurones ascending and descending in the central nervous system, (2) neurones connecting the two halves of any part of the central nervous system, and (3) neuronea connecting distant parts of the same side of the nervous system.
NEURO-PHYSIOLOGICAL BASIS OF ACTION 129 peduncles (brachium corgunctivum). If the three peduncles on each side are cut away the cerebellum can be removed. Fig. 32 shows the dorsal surface of the brain stem with the cere- bellum cut away. The three peduncles are marked 19, 20 and 21. The two hemispheres of the cerebellum are connected with each other by the vermis (Fig. 33, 18). The cerebellum has some very important cell masses, for example, the dentate nucleus, the FIG. 32.— Dorsal view of brain stem, showing the regions of the 4th ventricle, the corpora quadrigemina and the thalamus. 1, lateral funiculus (funiculus lateralis); 2 and 3, dorsal funiculus [respectively, funiculus cuneatus (Burdachi) and funiculus gracilis (Galli)]; 4, cJava4 23, tuberculum cuneatum, marking the reception nuclei of the dorsal funiculus; 5, ala cinerea; 6, auditory striae (striae medullares); 7, colliculus facialis; 8, part of roof of 4th ventricle (velum medullare anterius); 9, lingula cerebelli; 10, medial geniculate body (corpus geniculatum mediale); 11, pulvunar of thalamus; 12, stria terminalis; 13, caudate nucleus (nucleus caudatus); 14, pineal gland (corpus pineale); 15, septum pellucidum; 16, body of thalamus; 17 and 18, corpora quadrigemina, made up of the colliculus superior and colliculus inferior — each is continued into the thalamic mass by a brachium (brachium quadrigeminum superius and brachium quadrigeminum inferius); 19, superior cerebellar peduncle (brachium conjunctivum); 20, middle cerebellar peduncle (brachium pontis); 21, inferior cerebellar peduncle (corpus restif orme); 22, swelling marking nucleus of hypo- glossal (trigonum n. hypoglossi); 24, swelling marking medulla portion of reception nucleus of V nerve (tuberculum cinereum); dorsal median septum (fissura mediana posterior). The cavities above and below thalamus show the lateral ventricle. Note origin of 4th nerve (n. trochlearis) below inferior geniculate bodies.
9 130 PSYCHOLOGY 9 130 PSYCHOLOGY nucleus emboliformis, the nucleus globosus* and the roof nucleus (nuclei not shown). On the dorsal surface of the brain stem is shown the floor of the fourth ventricle (Fig. 32). The velum medullare anterius (Fig. 32, 8), together with the braehium con- junctivum, forms the roof of this ventricle. As has been pointed out, this ventricle is the remains of the embryonic medullary tube.
/O a FIG. 33. — Vertical median section of adult brain. 1, medulla oblongata; 2, pons (Varoli); 3, corpus mammillare; 4, hypophysis; 5, optic chiasm (chiasma opticus); 6, anterior commissure (commissura anterior); 7, knee of corpus callosum(genu corporis callosi); 8, septum pellucidum; 9, foramen of Monroe (foramen interventriculare Monroi); 10, massa inter- media; 11, third ventricle (ventriculus tertius) andthalamus; 12, pineal gland (corpus pineale); 13, aqueduct of Sylvius (aquaeductus cerebri Sylvii); 14, parieto-occipital fissure (fissura parieto-occipitalis); 15, corpora quadrigemina (lamina quadrigemina); 16, velum medullare anterius; 17, calcarine fissure (fissura calcarina); 18, vermis; 19, arbor vitse; 20, corpus medullare cerebelli; 21, fourth ventricle (ventriculus quartus).
It is continuous with the central spinal canal, and with the third ventricle above by way of the aqueduct of Sylvius (aquseductus cerebri Sylvii) (Fig. 33, 13).
The Cerebral Peduncles (Pedunculus Cerebri) and the Corpora Quadrigemina (Colliculus Superior and Colliculus NEURO-PHYSIOLOGICAL BASIS OF ACTION 131 Inferior). — Turning again to the ventral surface of the brain (Fig. 31), we see the two cerebral peduncles, 21 (pedunculus cere- bri), just above the pons. They are nearly covered by the over- lapping of the temporal lobes of the cerebral hemispheres. The cerebral peduncles are two large bundles of axones (right and left) connecting the parts we have just described with the parts yet to be described. They are close together when they emerge from the pons, but gradually separate as they pass upward, forming a recess which is occupied by the mammillary bodies (cor- pus mammillare) (Fig. 31, 10). These latter structures belong really to the forebrain. The term cerebral peduncle is very loosely used. In addition to the ascending and descending axones of which it is largely composed, there are to be found immedi- ately dorsal to the fibers the substantia nigra, a pigmented cellu- lar mass, and the tegmentum, in which are to be found many cellular masses serving as nuclei of reception for ascending and descending axones.
Fig. 33, which is obtained by splitting the brain symmetri- cally from front to back, thus dividing it into right and left halves, shows a medial view of the stem and mid-brain struc- tures. One finds there the corpora quadrigemina, four (two on each side of "the midline) small, but well-marked rounded masses (Fig. 33, 15). The lower two (right and left) are called the inferior colliculi, the upper two, the superior colliculi. They are much better shown in Fig. 32, 17 and 18. The two inferior col- liculi are continued into the medial geniculate bodies (corpus geniculatum mediale, Fig. 32, 10) and thence into the thalamus by heavy strands of axones called brachia. In a similar way the superior colliculi are continued into the lateral geniculate bodies (corpus geniculatum laterale). The inferior colliculi and the medial geniculate. bodies are a part of the auditory apparatus, while the superior colliculi and the lateral geniculate bodies are a part of the visual apparatus. The pineal gland (corpus pineale) is shown in Fig. 33, 12, and Fig. 32, 14.
The Thalamus and Related Structures.— We have already noted three of the structures belonging to the thalamic region, namely, the medial and lateral geniculate bodies and the pineal 132 PSYCHOLOGY 132 PSYCHOLOGY gland. The thalamus proper is an ovoid, couch-like mass which on its mesial surface forms the wall of the third ventricle. Fig. 33, 11, shows the thalamus in the left half of the brain. The massa intermedia (Fig. 33, 10) is a mass of gray matter connecting the two thalami. The medial view does not show the thalamus very well. Fig. 32, 16, shows the dorsal and lateral extension of this most important structure. Fig. 44 shows a coronal section through the thalamus. The dorsal surface of the thalamus usually shows four eminences, indicating the nuclear masses within. They are the anterior nucleus, the medial nucleus, the lateral nucleus, and the pulvinar. Only the pulvinar can be clearly seen in Fig. 32, 11. On the mesial surface near the thal- amus are to be seen the pituitary gland (hypophysis cerebri) (Fig. 33, 4). Note the ventral view of this same structure (Fig. 31, 18). The mammillary bodies (Fig. 33, 3) and the optic chi- asma (chiasma opticum) (Fig. 33, 5) are indicated both in the median section (Fig. 33) and in the ventral section (Fig. 31). These structures belong really to the telencephalon, as do the structures which are to be immediately described.
The Basal Ganglia. — Continuous with the thalamus but above it and lateral to it is to be found in each hemisphere the caudate nucleus (nucleus caudatus) (Fig. 32, 13, and Fig. 44, 13) and the lenticular nucleus (nucleus lentiformis) (Fig. 44, 10 and 12). They are two large cellular masses called the basal ganglia. Sep- arating the two cellular masses is an axone band called the in- ternal capsule (capsula interna) (Fig. 44, 20). These structures are shown only in a coronal section. The three structures to- gether are sometimes called the striate body. If the septum pellucidum were torn away (Fig. 33, 8) one would see first into the lateral ventricle, then the head of the caudate nucleus would appear as one of the boundaries of the cavity. The internal cap- sule is of vital importance. In it are gathered together in a nar- row space nearly all of the ascending and descending axones which connect the cortex and the lower structures.
The Corpus Callosum.— The corpus callosum, which is an NEURO-PHYSIOLOGICAL BASIS OF ACTION 133 enormous mass of commissural axones connecting the two cerebral hemispheres arches over the whole of the struc- tures we have just been describing. It is plainly marked 13 FIG. 34. — Convex surf ace of cerebral hemispheres as viewed from above. 1, longitudinal fissure (fissura longitudinalis cerebri); 2, occipital gyrus (gyri occipitales superiores); 3, postcentral gyrus (gyrus centralis posterior); 4, precentral gyrus (gyrus centralis anterior); 5, gyrus fontalis medius; 6, gyrus fontalis inferior: 7, superior frontal gyrus (gyrus frontalis superior); 8, frontal pole (polus frontalis); 9, superior frontal sulcus (sulcus frontalis superior); 10, precentral sulcus (sulcus precentralis); 11, fissure of Rolando (sulcus centralis (Rolandi)); interparietal euleus (sulcus interparie tails); 13,- parieto-occipital fissure (fissura parieto- occipitalis).
(Fig. 33, 7) and can also be seen in the coronal section (Fig. 44, 14). This system of axones can be seen quite easily if the two hemispheres are gently pressed apart: it appears as the floor of the great longitudinal fissure (Fig. 34, 1).
134 PSYCHOLOGY