Book - The Nervous System of Vertebrates (1907) 19
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Chapter XIX. The Neopallium
The evolution and general morphology of the cerebral hemispheres have been described in the last chapter and as full a description of the archipallium given as the limits of this book will permit. The neopallium, although it occupies the greater part of the mammalian hemisphere, is essentially dorso-lateral in position, lying between the pyriform lobe ventro-laterally and the hippocampal formation dorso-mesially. It is connected with the lower parts of the brain by large bundles of fibers which run down through the corpus striatum, forming the capsula interna (Figs. 162 and 166). By means of the corpus callosum the neopallial areas of the two hemispheres are connected with one another.
Structure of the Cortex
The neopallium everywhere consists of a thick internal layer of fibers and of a superfical layer of cells, the cortex. This cortical layer varies in thickness from about 1.70 mm. to about 3.50 mm. Its total volume increases enormously from lower to higher mammals on account of the increasing size of the hemisphere as a whole, and still more on account of the superficial folding in the higher forms. The cortex consists of several zones of cells of different forms which are to be recognized with certain modifications in all parts of the pallium. The typical structure of the cortex as seen in Golgi sections is illustrated in the accompanying Figure 171, which is combined from numerous figures of Cajal representing sections of the frontal and parietal cortex of the young child. As indicated by arabic numerals at the left of the figure, seven layers are distinguished according to the size and form of their cells and the disposition of their neurites.
1. Plexiform layer. This layer contains (a) small and medium sized cells with short neurites and (b) large horizontal cells whose neurites form tangential fibers of this layer. The layer also contains the terminal branches of the dendrites of pyramids and of other cells situated in deeper layers, and the end branches of the neurites of various cells of deeper layers.
2. Layer of small pyramids. This layer contains four types of cells: (c) small pyramids, each provided with a number of small basal dendrites and an apical dendrite which ascends to the plexiform layer, and having a long neurite which descends to the white substance; (d) large cells with short neurites; (e) small cells with short neurites which end in a very rich and dense arborization; (/) fusiform, ovoid, stellate or triangular cells without radial dendrites whose neurites form the "fibers of Martinotti" which ramify in the plexiform layer. Cells of the last type are found in all layers of the cortex and one is shown in the seventh layer in Figure 171.
Fig. 171 . Structure of the cerebral cortex. Explanation in the text.
Fig. 172. Diagram showing the probable course of impulses in the cerebral cortex. From Cajal (Nouvelles idees, etc.). A, small pyramid; B, large pyramid; C and D, polymorphic corpuscles; , terminal fiber from other centers; F, collaterals from the white substance; G, fiber bifurcating in the white substance.
3. External layer of medium and large pyramids. In addition to (g) the pyramidal cells, whose form is well shown in the figure, this layer contains several varieties of cells with short neurites, two of which are shown in the figure. One of these (h) is found in other layers as well. It has two sets of dendrites and a neurite with a great number of regular, smooth branches. The second (i) gives rise to a neurite which bears one or more pericellular baskets ending about the bodies of pyramidal cells.
4. Layer of small stellate cells and of pyramids, usually called the layer of granules. This layer contains a few small, medium and large pyramids similar to those in the adjacent layers, but the most numerous and characteristic elements of the layer are the small stellate cells. By reason of their great number and the distribution of their neurites these cells are very important elements of the cortex. They are of several varieties of which there are shown in the figure : (j ) cells whose neurites form long horizontal branches in the fourth layer; (k) cells whose neurites ramify in the third layer; (/) cells with ascending neurites with very numerous and exceedingly fine branches; and (m) cells whose neurites ascend to the first or second layer.
5. Deep layer of large pyramids. In various regions of the cortex are found discontinuous groups or islands of giant pyramidal cells, two of which are shown in the figure (n). The layer contains also cells with short neurites distributed to the fifth and sixth layers, or ascending to the first layer.
6. Deep layer of medium pyramids. This layer contains in addition to the pyramids (0) and triangular (p) and fusiform cells related to the pyramids, cells with short neurites of which several are shown in the figure (q,r).
7. Layer of triangular and fusiform cells. Some of the elements of this layer are true pyramidal cells ($), others fusiform or triangular cells (t) related to pyramids. The remainder are cells with short neurites (u), including cells (/) which give rise to fibers of Martinotti.
Fibers of the Cortex
Four categories of fibers are connected with the neopallium. (i) Afferent or exogenous fibers, coming to the cortex from other parts of the brain. Such fibers come from the sensory nuclei in the thalamus and perhaps from other sensory centers, such as the corpus quadrigeminum anterior and the nuclei of the VIII nerve. Such fibers therefore bring sensory impulses to the cortex, in which they end by widely spread arborizations. In addition to these fibers, collateral fibers of unknown significance rise into the cortex from the white matter. (2) Intracortical fibers. This group includes a great variety of fibers which serve to spread impulses in the cortex or to bring into relation more or less distant areas. There may be mentioned : (a) short neurites connecting the superficial layers with the deeper; (b) short neurites which connect more or less distant elements in the same or adjacent layers; (c) neurites which connect distant parts of the plexiform zone; (d) homolateral fibers of association which connect various parts of the cortex in the same hemisphere. (3) Fibers oj the corpus callosum. These connect the cortex of one hemisphere with that of another and are in part direct neurites of pyramidal cells and in part collaterals of fibers of association or of projection of one hemisphere which cross to the other. The place and manner of ending of the callosal fibers in man is not fully known, but it is thought that they are distributed to the motor and association areas. In some mammals they have been traced to endings in the motor sphere of the cortex. (4) Fibers of projection. These arise from the pyramidal cells and descend through the white matter to form constituent fibers of the internal capsule of the corpus striatum and end in various lower centers of the brain or spinal cord. They are therefore efferent fibers which carry impulses from the cortex and bring other parts of the nervous system under its influence.
Fig. 173. Scheme of long association tracts in the hemisphere. From Cajal (Nouvelles idees, etc.). a,b,c, pyramidal cells; d, terminal arborization; e, collaterals of the fibers of association.
Functional Areas of the Neopallium
The archipallium, as described in the last chapter, is devoted to olfactory and possibly gustatory functions. The neopallium is concerned with visual, auditory and general bodily sensations, with voluntary actions and with all the complex associational processes involved in our more highly organized activities, and in the formation and expression of ideas. But the whole of this large cortical area is not devoted indifferently to these several functions. It is shown by several independent methods of investigation that there exists in the neopallium a division of labor which is expressed by the term cerebral localization o) functions. Thus, in case of any disease or injury which produces a lesion of a part of the cerebral cortex, one or more bodily functions may be interfered with and the study of many cases has shown clearly that the functions affected depend upon the specific regions of the cortex injured. A sufficient number of facts of this sort have been collected from clinical observations, post-mortem examinations and surgical operations to render fairly certain and accurate the determination of the area of the cortex involved in case of a brain tumor, degeneration of cortical substance or other cause of disturbance. If the patient shows symptoms of disturbance in the functions of sight, hearing, bodily sensation, or voluntary movement including speech, certain specific areas of the cerebral cortex may be pointed out as the seat of the disease, and in the case of bodily sensation or movement certain subdivisions of the cortical area may be assigned to certain parts of the body.
Fig. 174. Scheme of commissural and projection fibers of the cortex. From Cajal (Nouvelles idees, etc.). A, corpus callosum; B, anterior commissure; C, pyramidal tract.
These clinical observations on man are supported by experimental investigations on animals in which either certain areas of the cortex in the living animal are directly stimulated and the effect noted, or the degenerations produced by the extirpation of certain cortical areas are studied by the method of Marchi. These investigations combined with the study of degenerations in human brains where the symptoms have been recorded during life, have led to the mapping out of functional areas on the cortex and to the description of the course of the tracts connected with those areas. Now the histological study and comparison of the areas thus marked out shows that in the normal brain certain differences exist between the different regions of the cortex and these differences are in some cases sufficiently marked to be of assistance in determining the limits of the various areas. Thus the cortex in various regions differs in total thickness; in the relative thickness of its layers, in the presence, number and arrangement of giant pyramids and other cells; and above all, in the character of fibers connected with it. The skilled observer can distinguish between sections from various cortical areas. In the case of the cortical area for hearing, the thickness alone is sufficient guide and in the visual area the tangential fibers connected with it are so well marked that the limits of the area may be seen in dissections with the naked eye.
The differences in the fiber tracts connected with the several areas give by far the most simple and direct and probably the most accurate means of mapping them out. The clinical and pathological data come into account here, but the most illuminating studies are those based upon the order of myelinization of the fiber tracts (method of Flechsig). Flechsig has shown that in the developing human brain certain fiber tracts produce their myelin sheaths earlier, others later, and that the order of myelinization is measurably regular and constant. Certain practical advantages in tracing fiber tracts were gained by this discovery. The tracts which are myelinated early may often be traced with great ease in early stages before adjacent tracts have received their myelin. When the tracts which are myelinated early are fully known in early stages of development the tracts which become myelinated later may be studied with greater certainty. The great advantage for the study of cerebral localization, however, lies in the fact that the majority of fibers connected with any nerve center or engaged in carrying impulses of the same category, become myelinated at the same time. If, then, the fibers which go up to the cortex from the centers for hearing, for example, become myelinated at the same time, the area of the cortex concerned with hearing will be indicated by the distribution of this tract when it is first myelinated and can be traced most easily. Furthermore, the production of myelin seems to be the result of the beginning of functional activity of nerve fibers and the" myelin appears first near the cells of origin and is progressively formed toward the end of the fiber and so indicates the direction in which the impulses travel. Thus, when a sufficient series of developing brains are studied this method should give three classes of facts: first, the course of the fibers engaged in a specific function and the cortical area to which they are related; second, the order in time when various nervous pathways begin to function; and third, the direction in which impulses are carried by given fiber tracts.
The Senso-Motor Areas
The afferent or sensory fibers which come to the neopallium from lower brain centers end in certain regions, and from these regions alone (Flechsig) arise the great majority of the efferent or motor fibers which carry impulses to the motor centers of the brain and spinal cord. Hence these regions are given the name of the senso-motor areas. Three sensory centers are recognized, the visual, the auditory and the somaesthetic areas. The last is the area for general bodily sensations. Since motor tracts descending from the visual and auditory fields are not well known, a general area corresponding roughly to the somaesthetic area is commonly known as the motor area. The only way of describing the limits of these areas is by means of the superficial sulci and gyri of the brain. Their position will be described here in a general way; for their exact boundaries the student must be referred to the original articles dealing with the subject.
Fig. 175. The primordial areas in the cerebral hemispheres, lateral surface. From Flechsig (Einige Bemerkungen u.s.w.). The numerals indicate the order of myelinization of the several areas. The areas 9-13, although myelinated early have no projection fibers. The areas 1-8 belong to the primary sensory areas (compare Figs. 179 and 180).
Fig. 176. The primordial areas in the cerebral hemispheres, mesial surface. From Flechsig. See Fig. 175.
1. The somaesthetic area. This includes the central gyri, the lobulus paracentralis and part of the adjacent frontal gyri and of the gyrus fornicatus; that is, a part of the lateral and mesial surfaces of each hemisphere in the middle region. The extent of the area is indicated in the accompanying Figures 175 and 176. This area receives fibers from the chief sensory nucleus of the thalamus and perhaps from other centers' of the general cutaneous apparatus (cf. p. 259). With respect to the time of myelinization these fibers are divided into three groups. The fibers of the first group receive their myelin at the beginning of the ninth month of foetal life and are the first fibers entering the neopallium to become myelinated. Part of the fibers of the olfactory tract are myelinated somewhat earlier. The fibers which are first myelinated carry impulses from the limbs and this accords with the evident importance in the early life of the infant of the tactile impressions received through the limbs. The fibers carrying impulses from the trunk and head are myelinated later.
2. The visual area. This occupies a small part of the lateral surface and a large part of the mesial surface in the occipital region. The fibers from the optic centers in the thalamus (p. 261) receive their myelin just later than the greater part of the fibers of the olfactory pathways and earlier than the last group of somaesthetic fibers.
3. The auditory area. This area on the lateral surface of the temporal lobe close to the island of Reil is better defined than most other areas, since its cortex is much thicker than that of the immediately adjacent regions. Its afferent fibers are myelinated soon after those of the visual field.
4. The gustatory area. As already suggested in the previous chapter, the gustatory center is closely related to the olfactory. Injury to the cortex or fiber tracts close to the splenium of the corpus callosum in one hemisphere leads to loss of taste on the opposite half of the tongue. Whether the taste center is located in this area of the cortex (area 6 of Flechsig's figures) which is continuous with the subiculum cornu Ammonis, or in the hippocampus itself is uncertain. When the gustatory connections in lower vertebrates are taken into account it seems very probable that gustatory as well as olfactory sensation is provided for in the archipallium and that both come into relation with the mechanism for conscious and voluntary actions only through the association centers.
5. The motor area. This is practically co-extensive with the somaesthetic area. In this area the pyramidal cells reach a greater development than elsewhere and the large pyramids are especially numerous. In those areas in which arise fibers which run to the lower part of the spinal cord many of the pyramidal cells are of gigantic size (e.g. the giant cells of the lobulus paracentralis, the center for the lower limb). The motor fibers always develop myelin later than the sensory fibers of the corresponding area. From the whole motor area arise fibers which descend through the internal capsule and cerebral peduncle and continue through the pons and medulla oblongata into the spinal cord. In the internal capsule they make up the knee and two-thirds of the posterior limb and they constitute about three-fifths of the cerebral peduncle. In the base of the midbrain and pons many of the fibers terminate in relation with the motor neurones of the III, IV, V, VI and VII nerves, most of them crossing to end in the nuclei of the opposite side. Many fibers end also in the nuclei of the pons. Those fibers which pass through the pons form in the medulla oblongata the well-known pyramidal tracts. From these, fibers are given off to the motor nuclei of the IX, X and XII nerves and at the posterior end of the medulla oblongata the two tracts enter into the pyramidal decussation. The decussation is not complete but the smaller part of the fibers continue into the cord on the same side, forming the ventral cerebro-spinal fasciculus. The crossed fibers together with a small number of the uncrossed fibers form the lateral cerebrospinal fasciculus. Both of these bundles decrease in size as they descend the spinal cord, the ventral bundle being used up in the cervical and thoracic region and the lateral bundle gradually diminishing but extending even into the sacral cord. The fibers end in relation with the motor neurones of the spinal nerves. Part of the fibers of the ventral bundle cross to the opposite side and since the lateral bundles also contain uncrossed fibers it is probable that throughout the whole length of the spinal cord there are both direct and crossed connections of the cerebral cortex with the motor nerves.
6. The areas of association. When the areas which have been described are myelinated there remains two-thirds of the cortical area without myelinated fibers. At the same time the projection tracts of the cerebral cortex seem to be fully formed. Although it is quite possible that a certain number of projection fibers may receive their myelin after this time and that some of such fibers may be connected with parts of the cortex not included in the above areas, it is clear that these are the supreme projection areas of the cortex. Although the method of Golgi and the degeneration methods give evidence that projection fibers arise from the whole cortex, the method of Flechsig is the more reliable for this question and it must be regarded as clearly established that the number of projection fibers connected with other parts of the cortex is small compared with the number connected with the senso-motor areas above described.
The remaining two-thirds of the cortex is intercalated between the several senso-motor areas so that each of them is separated by a considerable space, from the others. This whole area constitutes, according to Flechsig, the association centers and it may be divided into three main fields, the anterior, middle and posterior association fields (Fig. 179). That these fields are primarily related to the senso-motor areas is shown by the fact that the myelinization of the cortex spreads from the senso-motor areas to the cortex bordering on them. A comparison of Figures 175 and 176 with Figures 177 and 178 shows that around the senso-motor areas a number of border zones have been myelinated while the central zones of the association fields remain without myelin. This seems to indicate that when the senso-motor areas become functional the association areas immediately adjoining them first come into relation with them, and the central parts of the association fields become functional last. A difference in function, then, is probably to be attributed to the border zones and the central zones of the association fields.
Fig. 177. The primordial areas and the border zones of the association fields, lateral surface. From Flechsig. The figure represents the extent of myelinization in the hemisphere of a newborn infant 54 cm. in length. The primary sense areas and a part of the border zones are myelinated and the numerals indicate the order in which they receive their myelin. The anterior and posterior association fields stand out sharply as light areas. The middle association field is exposed to view by drawing aside the temporal lobe.
Fig. 178. The primordial areas and the border zones of the association fields, mesial surface. From Flechsig. See Fig. 177.
Fig. 179. Diagram of lateral surface of hemisphere showing localization of functions.
Fig. 180. Diagram of mesial surface of hemisphere showing localization of functions.
By association fields must not be understood areas in which the functions of association are carried out without the aid of the senso-motor areas. This would be physically impossible and the term correlation centers would more truly express the function of the association fields. In other words, the function of the association centers is to correlate the actions of the sensomotor centers. The cells in the senso-motor areas are by no means all connected with projection fibers; only a few of them give rise to such fibers. The remainder give rise to association fibers of greater or less length. The shorter ones serve to connect the layers in the same gyms or to carry impulses to adjacent gyri. The longer ones carry impulses to more distant parts of the cortex. It appears that these fibers do not go from one senso-motor area to another, but that the several senso-motor areas are brought into relation through the association centers. The fibers reach first to the adjoining border zones of the association fields and later to the central zones and even to more distant association centers. The longer association fibers enter into the formation of certain long association tracts of which there may be mentioned : (i) the fasciculus longitudinalis superior or arcuatus, apparently connecting the occipital and frontal lobes; the fasciculus longitudinalis inferior, connecting the occipital and temporal lobes; the cingulum, connecting the hippocampus and perhaps other olfactory nuclei with the lower gyri of the frontal lobe; the fasciculus uncinatus, connecting the temporal lobe with the same region of the frontal lobe; and the fasciculus occipito-frontalis (tapetum) which Dejerine believes consists of fibers arising in the frontal lobe and ending in the occipital lobe. In addition to these -tracts which connect distant lobes, within each lobe are numerous association bundles, such as the calcarine, vertical, and transverse bundles in the occipital lobe. These various association bundles are very complex, consisting in most cases of fibers running in both directions and of fibers which enter and leave the bundles at various points of their course. Their grouping into bundles is due merely to mechanical conditions arising from the form of the brain. The greater part of the corpus callosum fibers go to the anterior and posterior association fields; the middle field receives few commissural fibers.
In order to understand the functions of the association centers it is necessary to define more exactly the functions of the sensory areas. The phenomena of clear and sharp sense impressions is dependent upon the sense areas. Disease or injury to one of these centers interferes with the clearness and definiteness of the sen-* sations with which that area is concerned. At the same time the memory of such sense impressions and percepts formed from several kinds of sense impressions may remain intact. Also, according to Flechsig, the perception of the spatial and temporal relations between sense perceptions depend upon the sense areas. Thus the clear recognition of the form of an object when felt by the hand depends upon the proper functioning of the hand and arm portion of the somaesthetic area, while the memory picture of the same object made up from previous sense impressions (tactile, visual, etc.) is a function of some of the association centers and may be preserved when the somaesthetic area is diseased.
As the sense areas send fibers into the adjoining border zones of the association fields the participation of these zones will provide for the combination of simple sense impressions into perceptions of slight complexity. Thus general images of form based upon the examination of various objects by the hand may be localized in the border zone adjoining the sense area for the hand. In some such way the border zones stand in relation with the several sense areas and provide for relatively simple association of sense impressions from nearly related regions of the body. More complex images of the form of objects dependent upon the combination of tactile and visual impressions, for example, require the cooperation of association centers which receive fibers from both somaesthetic and visual areas. The order of myelinization of association centers corresponds to the order of development of more and more complex actions and mental states in the child. The central zones of the association fields, then, serve for the higher psychic states and more complex processes of thought. The specific functions of the three association fields are determined in large part at least by their position with reference to the senso-motor areas. The middle association field (island of Reil) situated as it is between the auditory area and that part of the somaesthetic area which receives sensation from the lips, tongue, throat, etc., is chiefly the association center for speech. The anterior and posterior fields have much more complex relations and functions. The posterior field, situated between the somaesthetic, visual and auditory areas, receives from those areas impressions concerning the external world. The functions of this field are to construct external objects from the several kinds of sense impressions and to form ideas concerning the relations of objects and physical processes to one another and to the self. In a word, the objective relations of the individual and all those processes which we commonly call "intellectual" are the peculiar province of the great posterior association field. That this is at least in general the true interpretation of this field is evidenced by the mental symptoms in cases of disease affecting the posterior association field. The most common phenomena met with are various forms of loss of memory and of the power of association. If the sensory areas are not affected the perception of sense impressions by the touch, sight and hearing is not impaired, but these impressions can not be combined into objects which are recognized as previously experienced. The individual presents to the observer the phenomena of mind-blindness, mind-deafness and the like, while he himself loses the power of connecting his several impressions into orderly experience. Whatever set of associations are thus affected the corresponding set of sense impressions cease to interest and finally cease to attract the attention of the individual and he appears listless with regard to things which once interested him, or is unable to recognize objects, to give the proper names to things, to remember appointments, etc. When larger areas of the posterior field become affected these symptoms become more general and the patient loses his interest in the external world and practical affairs. He may, however, retain his interest in his own personal relations, his self-respect may be perfectly preserved, and so far as his powers of intelligent action enable him, he may be entirely true to his personal duties and engagements. The posterior association field deals then with order and relation in the external world, with recognition, memory and imagination, with judgment and the weighing of processes and events.
The anterior association field lies in proximity to the somaesthetic area, but removed from the other sensory areas. It would receive in common with the posterior area the impressions due to contact with external objects and to the movements of the body, limbs, organs of speech, etc. Although the distant connections of the frontal lobe are not well understood, it seems clear that fibers from the visual and auditory areas are of subordinate importance, while association bundles from the olfactory (and gustatory) centers are next in importance to those from the somaesthetic areas. In the light of comparative neurology it is more than possible that all sensation from the viscera reaches the archipallium and comes to play a part in conscious states through the association tracts to the frontal lobes. If so, this would strengthen the view already expressed by Flechsig that the impulses which enter the frontal lobes have to do especially with experiences of the individual and hence the anterior association field is concerned with subjective states, and with the emotions, with action and with the will. Here are to be sought the mechanisms corresponding to the bodily states which accompany or constitute the emotions, and those connected with attention and apperception. Certain bodily conditions, such as muscular tension, are connected with attention and active apperception, and the association centers which deal with these are related to the somaesthetic and somatic motor area. Here, also, clinical observation supports the reasoning from anatomical data. In cases of disease of the frontal lobes the symptoms which appear during life involve loss of appreciation of the individual's personality and of the value of things to him. Self-depreciation and lack of confidence or the opposite extremes, incapacity for moral and aesthetic judgment, uncertainty of action and lack of will, are common in such persons. Their self-control suffers and under the influence of excitement their conduct becomes immoral or criminal. The anterior association field has to do, then, with ideas of the individual's personality and with the appreciation of his personal relations. For conduct in the full sense, that is for moral conduct, there is required the normal functioning of both anterior and posterior association fields; since both objective and- subjective relations must be considered, both judgment and will are involved. For moral conduct the individual must respond to both the external or objective and the individual or subjective factors in his situation, and the perfection of the response depends upon the grade of organization of the association centers and the balance between them.
The Evolution of the Neopallium
The general subject of the origin of the neopallium has been considered in the last chapter, but a certain interest attaches to the order and grade of development of its various parts. The direct data for the study of these subjects are very meager. The collection of such data would require the study of cerebral localization in the various orders of mammals together with the study of habits and the grade of organization of intelligent action. Such studies are not yet complete enough for this purpose but the facts of localization in the human brain give some indications of the probable course of development. First, the facts that the senso-motor areas are everywhere separated by association areas and that myelinization proceeds from the sensomotor areas into the association areas warrant the inference that the association areas have been differentiated from the borders of already existing senso-motor areas. The senso-motor areas must have occupied the greater part or the whole of the neopallium in lower mammals and the greater size of the hemispheres in higher mammals is due chiefly to the expansion of the association areas. Second, the position of the several senso-motor areas indicates the order of development in the neopallium. The phylogenetic and ontogenetic development of the several main regions of the forebrain show that the neopallium began its history at the front end of the forebrain and that it has expanded backward. Consequently the centers which were first developed must now be found in the posterior part of the hemisphere and those which were last developed, in the anterior part. The visual centers, then, which occupy the extreme occipital pole, were first formed, next the auditory which are depressed into the temporal lobe by the development of the association centers, and last the somaesthetic and somatic motor area. It was pointed out in the last chapter that a forerunner of the neopallium is probably present in the fish brain in the form of a tract from the tectumopticum to the forebrain, and this accords well with the conclusion that the visual center was the first to be developed in the cortex. The order indicated by the position of the organs in the cerebral cortex of man accords also with the order of importance of the sense organs in phylogenetic history. While the cutaneous system is the oldest it is also the least specialized. The eye is the first special somatic sense organ in vertebrates and the auditory system followed it. It may easily be seen that these special sense organs which give knowledge of objects at a distance would be of greater value for the development of a higher correlating center such as the cortex than the cutaneous system. Although the cutaneous system was the last to reach the cortex its center hi the neopallium has become larger than both visual and auditory combined. The relative areas are approximately proportional to the number of peripheral sensory fibers. The great importance of the somaesthetic area in apes and man is connected with the development and mobility of the limbs; especially with the high organization of the hand as a grasping and tactile organ, and the use of tools. It may be supposed that while the senso-motor areas were being developed, association centers made their appearance between them, and that the posterior association center is therefore older than the anterior. The expansion of the association centers has helped to determine the position of the several areas in man. Indications of this are seen in the shifting of the visual area from a position about half on the mesial surface and half on the lateral surface in the apes to a position almost wholly on the mesial surface of the hemisphere in man, and also in the position of the auditory area in the temporal lobe. Finally the anterior association field has developed from the front border of the somaesthetic area and is relatively small in the apes and probably absent in the lower mammals. The development of this field seems to be directly proportional to the grade of self- consciousness and in the several races of men, to the grade of civilization. So, the full appreciation of the self has probably been the last and highest factor in the development of individual and social conduct.
The development of the several areas demands an increase in cortical surface. This is secured by surface foldings which form the gyri and sulci. Such foldings took place (i) within the sensory areas themselves, serving to increase their surface; (2) at the border of the sensory areas and so forming the boundaries between sensory and association areas; and (3) within the association fields themselves. The deeper and more constant furrows belong to the first and second groups, since the furrows in the association fields appear much later than the others. After the fundamental furrows were formed they were mechanically prolonged, branched and bent until it is difficult to compare the furrow patterns of the brains of different mammals.
One other consideration concerning the association centers should be touched upon in closing this book, namely their significance for education and morals. The object of human knowledge is the world in its multitudinous forms and man's manifold relationships in it. The knowledge already acquired when taken in comparison with the simplicity of the child's brain and knowledge has become enormously complex. It is so complex that when it is systematized and divided and subdivided many times, the average man can grasp only one small subdivision. How many of these shall the child entering school today learn to understand, with how many can he become reasonably familiar, in how many will he be able to add to human knowledge? This will depend chiefly upon the number of related facts which come to his notice during the plastic period of his brain. If he is early brought into contact with the world of matter and of life under many aspects, a corresponding number and variety of association tracts in his brain will be developed and fixed through use and will be ready for the correlation of the experience of later years. Either in the schoolroom, on the playground or in vacation time a richness of experience must supply the association centers with the condition for their high organization. Without this there can be in later years no richness of mental life and no great power of research, invention or constructive statesmanship. So on the moral side, the realities of human relationships must come into the experience of the youth during the period of plasticity of his brain if he is to rise to the higher planes of moral life.
Demonstration Or Laboratory Work
- Dissect the brain of a cat, dog, sheep or man, working out the relations of the neopallium to the archipallium and corpus striatum. With the aid of figures in a larger text-book or atlas trace the limits of the neopallium in man and the chief gyri and sulci and the location of the senso-motor and association areas.
- As far as time and material permit, the structure of the cortex, the course of projection and commissural fibers and the arrangement of association bundles may be studied in Golgi or Weigert sections of a mammalian brain.
Beevor and Horsley: A record of the results obtained by electrical excitation of the so-called motor cortex and internal capsule in an Orang-Outang. Phil. Trans. 1890-91.
Beevor and Horsley: An experimental investigation into the arrangement of the excitable fibers of the internal capsule of the bonnet monkey. Phil. Trans. 1890-91.
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