Book - The Nervous System of Vertebrates (1907) 18
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Chapter XVIII. The Evolution of the Cerebral Hemispheres
The cerebral hemispheres of man are the largest and most complex part of the nervous system. They are also proportionately larger and more complex than in animals. The degree of development of the cerebral hemispheres is correlated with the mode of life of the animal. The greater size and the complexity of internal structure of the hemispheres in man are a measure of the degree of organization of his activities, the perfection of his adjustment to the manifold aspects of his environment, and the correlation of experience which makes such adjustment possible. The cerebral hemispheres constitute a mechanism whose structure is determined by all the experience of the race and of the individual, and in the working of whose minute parts is found the means of directing every one of the more complex activities of the man. In order to understand this highly organized mechanism every means of study must be employed, and nowhere has the comparative method proved more useful than here. The differences between the human cerebrum and that of lower animals would seem at first sight to be so great as to make intelligent comparison impossible. In fact, as will appear below, the most fundamental parts of the cerebral hemispheres of man are present and have the same structure in all vertebrates, while for the study of the most highly specialized parts comparison with the various orders of mammals is very fruitful.
The hemispheres of man are divisible into three widely different parts. The part by which the hemispheres are directly connected with the rest of the brain is a thick bi-lobed mass consisting of collections of nerve cells pierced by numerous large bundles of fibers and is known as the basal ganglion or corpus striatum. It is by way of the corpus striatum that the chief fiber tracts connecting the hemispheres with the rest of the brain pass to or from the thalamus. Lying in front of and below the corpus striatum and forming part of the lower and mesial wall of the hemispheres at the anterior end are the olfactory bulb and olfactory lobe. On the ventral surface of the striatum is the nucleus amygdalae which is continuous caudally with the pyrijorm lobe and the hippocampus in the temporal region of the hemisphere. These several structures, together with the fornix and hippocampal commissure constitute the second main portion of the hemisphere, and may be spoken of collectively as the central olfactory apparatus. All the rest much the greater part of the cerebrum is concerned with sensory impulses from the external world which come from various parts of the body including the special sense organs of sight and hearing; with the correlation of these impulses with one another and with habitual tendencies produced by previous actions; with voluntary impulses se"nt out to arouse, direct or inhibit actions in response to stimuli; with sensations; and with thought processes. This portion of the cerebral hemispheres may be spoken of as the somatic pallium. The phylogenetic history of these three portions of the forebrain has formed one of the most obscure chapters of comparative morphology. If it is possible to frame a connected account of the evolution of these structures, .it will render the study of the human cerebrum simpler and its relations more intelligible.
In order to gain a clear view of the vertebrate forebrain it will be necessary to begin with the lowest classes and give as concise an account as possible of the centers and fiber tracts and their functional relations in one class of vertebrates after another, endeavoring to fix with as great certainty as possible the homology of the most important structures and to harmonize those inconsistencies which arise from differences of nomenclature or interpretation of known facts. In such a survey the guiding principle must be functional relationship. When a given center or fiber tract is clearly recognized anatomically, the questions must be asked, what is its function; with what other centers or fiber tracts is it related; with what kind of impulses is it concerned?
The Forebrain of Cyclostomes
Consists of paired lateral lobes and of a mesial portion connected with the diencephalon.
The lateral lobes are rounded and are divided into anterior and posterior portions by a slight groove on the lateral surface, nearly vertical in position. If the brain be cut into halves in the median sagittal plane and one half looked at from the mesial surface (Fig. 145) the relation of all the parts can be better seen. In such a hemisection there is seen in the dorsal wall of the diencephalon the thick nucleus habenulae and opposite it ventrally the optic chiasma and the decussations of other tracts. Behind this is the depressed hypothalamus. The limit between the diencephalon and telencephalon is roughly indicated by a line drawn between the optic chiasma and the nucleus habenulae. In front of this line the median or third ventricle extends forward through the whole length of the telencephalon (cf. Fig. 8). It is bounded laterally by thick walls but dorsally and ventrally the walls are much thinner. Just in front of the optic chiasma the floor of the ventricle is suddenly depressed to form the preoptic recess. In front of this recess is the point corresponding to the lower edge of the neuropore in the embryo. From this point forward and upward the brain wall is formed by the closing of the neuropore in the median line. As this is regarded as the end of a closed tube the front wah 1 of the median ventricle is called the lamina terminalis. The lower portion of the lamina terminalis is thickened by the anterior commissure, and the upper portion of it is also thickened by a transverse band of fibers known as the olfactory decussation. Immediately above the olfactory decussation is seen a short triangular projection of the ventricle which marks the point at which the neural tube remained longest in connection with the ectoderm. This was probably at the upper border of the neuropore and the little sac is called the recessus neuroporicus. From the recessus neuroporicus back to the nuclei habenulae the roof of the median ventricle is membranous. This is a part of the true brain roof and does not belong to the lamina terminalis. There is no such thing as a pars supraneuroporica of the lamina terminalis. In the front part of the forebrain the median ventricle expands laterally into the cavities of the lateral lobes. These cavities are the lateral 'ventricles and each has an anterior and a posterior branch corresponding to the two parts of the lateral lobes. These must not be confused with the anterior and posterior horns of the lateral ventricles in man, since both together correspond more nearly to the anterior horn in man. The connection of the median with the lateral ventricle of each side is known as the foramen of Monro.
Fig. 145. A diagram'of the fiber tracts in the forebrain of a cyclostome, Lampetra Wilder*. The mesial surface of the right half of the forebrain and interbrain is drawn. The deeper shading indicates the wider parts of the ventricle and the dark oval opening is the foramen of Monro. The fiber tracts and parts of them which lie farthest laterad are'drawn in the darkest shade.
Into the anterior half of each lateral lobe the olfactory nerve enters and its fibers are distributed to all parts of the anterior half of each lobe. This anterior part of the lateral lobe is therefore the olfactory bulb. It consists of a great number of slightly differentiated cells (Fig. 94) from which arise the fibers of the olfactory tract. These fibers pass back into the posterior portion of the lateral lobes, to the whole extent of which they are distributed. These are the olfactory lobes, including the equivalent of the nucleus thaeniae of higher forms. The vertical groove in the lateral wall separating the olfactory lobes from the bulbs is the only external indication of the olfactory tract, which is usually recognizable externally in other vertebrates. A part of the olfactory tract fibers do not pass back on the same side but cross to the opposite side in the upper part of the lamina terminalis, forming the olfactory decussation. These fibers then enter the dorsal part of the wall of the median ventricle and do not go to the lateral lobes. This lateral wall of the ventricle is directly continuous caudally with the lateral wall of the diencephalon and through the foramen of Monro with the caudal wall of the olfactory lobe. The nucleus which occupies this area is the epistriatum (Fig. 146). The olfactory tract, then, goes in greater part to the olfactory lobe of the same side and in lesser part through a decussation in the lamina terminalis to the epistriatum of the other side.
Fig. 146. An oblique section through the inferior lobes and forebrain of Lampetra. Ill, median ventricle; a.o., olfactory lobe; d.p., postoptic decussation; e, epistriatum; ep., epiphysis; tr.e-s., neurites of epistriatum cells to striatum; tr. l-b., tractus lobo-bulbaris; tr.o., tractus opticus; tr.o-h., tractus olfacto-habenularis; tr.s-th., tractus strio-thalamicus; tr.th-sae., tractus thalamo-saccus; sac., saccus vasculosus.
From the olfactory lobe two tracts arise. One passes internal to the optic tracts and enters the inferior lobes of the diencephalon, the tractus olfacto-hypothalamicus. The other goes upward and backward through the epistriatum to the nucleus habenulae, where the larger part of its fibers cross to the opposite nucleus, forming the commissure, habenularis. This is the tractus oljactohabenularis. In addition to fibers of the olfactory tract which end in the epistriatum an equally large tract comes to it from the hypothalamus by way of a decussation behind the chiasma. This is an important tract the equivalent of which is probably found in all vertebrates, usually in close relation with the tractus striothalamicus. It should be recognized as an independent tract under the name of the tractus lobo-epistriaticus. The epistriatum is characterized by pyramidal cells whose dendrites are studded with little knobbed spines. The cell-body lies next the ventricle and the neurite arises from the basal part of a dendrite and proceeds away from the ventricle. The neurites descend through the lateral wall of the median ventricle and end in the ventral half of the same wall, which constitutes the corpus striatum. The cells of the striatum are irregularly arranged bipolar or stellate cells whose neurites form the tractus strio-thalamicus, which ends in the central gray of the thalamus. Surrounding the preoptic recess is a layer of cells of very primitive character forming the nucleus praeopticus. The neurites from this nucleus in part run dorsad to enter the tractus olfacto-habenularis and in part run back over the chiasma into the hypothalamus. The fibers which constitute the anterior commissure have not been definitely traced.
This is the most primitive type of forebrain that has been studied. It is concerned with olfactory impulses and possibly with gustatory impulses which may reach the epistriatum by way of the hypothalamus. It will be seen as the account proceeds that the centers and fiber tracts which have been described are preserved in higher vertebrates.
The Selachian Forebrain
The forebrain of the primitive selachians (e.g. Heptanchus, Fig. 2) and of the Holocephali (Chimaera, Fig. 7) is rather slender and elongated. That of the more specialized forms (e.g. Squalus acanthias, Figs, n and 147, Scyllium, Raja, Torpedo, etc.) is much more massive and compact in form. The difference is chiefly a difference in size due to the greater development of certain parts of the brain in more specialized selachians. In both, the walls of the median ventricle are essentially as in cyclostomes except that the ventro- cephalic and the front part of the lateral wall are enormously thickened. The foramen of Monro leads laterally from the front end of the median ventricle into the lateral ventricle which traverses the thick lateral mass and continues forward through a more or less elongated olfactory tract into the olfactory bulb (Fig. 8). This is the typical form for the olfactory bulb in vertebrates. In cyclostomes the bulb has been pushed out to the side and backward by pressure from the large buccal funnel, and the side wall of the forebrain (olfactory lobe) has been folded outward and backward so as to form a sort of posterior horn to the lateral ventricle which is quite peculiar to the cyclostomes.
The great mass at the front of the brain is composed of two parts the limits between which are readily seen in Figs, n and 147. Above and in front of the foramen of Monro the median ventricle is produced into a small pointed sac, the recessus neuroporicus. This meets a fibrous strand from the pia which descends through a narrow canal from the dorso-cephalic surface of the forebrain. This canal marks the dorsal border of the lamina terminalis and the mass lying in front of it is formed by a thickening of the lamina terminalis and of the adjacent wall, between the bases of the two olfactory tracts. As indicated in Figures 8 B and 9, the great growth of the front walls of the lateral lobes has resulted in their apposed mesial faces fusing together so as to give the appearance of an enormously thickened lamina terminalis. The preservation of the anterior branch of the lateral ventricle and of the canal from the dorsal surface serves to show the primary form of this region. At either side of the recessus neuroporicus in Figure 8
Fig. 147. A diagram of the fiber tracts in the forebrain of a selachian. The mesial surface of the right half of the brain of Squalus acanthias is drawn and the fiber tracts projected upon it. The course of the tracts is taken chiefly from the description by Kappers, but the work of Catois, Edinger, Houser, Locy and others has been considered.
B is seen a ridge projecting into the lateral ventricle, the regio untinata. This represents the front wall of the brain immediately adjacent to the lamina terminalis. The great mass which has been formed by the expansion, thickening and bending upward of the front walls of the brain between the olfactory bulbs will be called the mesial olfactory nucleus.
The lateral walls have also grown up, thickened enormously and fused together over the ventricle to form the massive roof seen in Figure 147 behind the canal leading to the recessus neuroporicus. The basal portion of the lateral wall is occupied by the corpus striatum. The outer layers of the lateral wall and of the thick roof constitute the lateral oljactory nucleus. The inner surface of both the striatum and the lateral olfactory nucleus is covered by a gray layer rich in small cells which bounds the ventricle both ventro-laterally and dorsally. This is the epistriatum. The course of the fiber tracts will show the functional significance and homology of the various parts of the selachian brain. The olfactory tract spreads through the whole anterior and lateral wall and ends in the mesial and lateral olfactory nuclei. From each of these nuclei a tract runs to the hypothalamus. The first gathers from all parts of the mesial nucleus, curves downward and backward in the thick front wall, runs back near the mid- ventral line and joins the tractus strio-thalamicus. This tract should be called the tractus olfacto-hypothalamicus medialis. The second tract collects from the lateral olfactory nucleus, runs downward and backward through the lateral wall, passes over the optic chiasma, forms a decussation behind the chiasma and ends in the hypothalamus. This tract has usually been called the pallial tract because the nucleus from which it arises forms the roof of the forebrain. This nucleus, however, is the place of ending of olfactory tract fibers and is hence merely a part of the olfactory lobe. The tract in question should therefore be called the tractus oljacto-hypothalamicus later alls. From the lateral part of the lateral olfactory nucleus another tract arises which runs along the dorsal border of the narrow lateral wall of the posterior part of the forebrain and enters the nucleus habenulae. Here the tract decussates with its fellow to form the habenular commissure and ends in the nucleus habenulae of the opposite side. This is the tractus olfacto-habenularis and corresponds to the tract of the same name in cyclostomes. That part of the lateral olfactory nucleus from which it arises may be called the nucleus thaeniae. The two tracts to the hypothalamus correspond to the one in cyclostomes, the existence of a separate medial tract in selachians being due to the extraordinary development of the olfactory apparatus.
The commissures in the forebrain of selachians differ in some respects from those in other vertebrates. In the floor (lamina terminalis) in front of the recessus praeopticus is the anterior commissure which contains two sets of fibers : one coming from the tractus strio-thalamicus and the other from the lateral olfactory nucleus and both ending in the deeper parts of the lateral wall, the epistriatum. In the roof is a large commissure which contains two kinds of fibers. The greater part of it consists of a decussation which is said to be composed of fibers from the dorsal part of the mesial olfactory nucleus and not of olfactory tract fibers. The fibers of this decussation end in the regio uncinata and in the epistriatum. In addition to this a direct tract arises in the mesial nucleus and ends in the epistriatum of the same side. These tracts in the selachian brain are new as compared with the brain of cyclostomes. It must be held clearly in mind that these fibers form the third link in a chain of which the olfactory nerve and the olfactory tract form the first two links. These fibers therefore constitute a tractus olfacto-corticalis. The nucleus which receives these fibers is a center of the same grade as the olfactory cortex of higher vertebrates. That part of the epistriatum which forms this nucleus must be called the olfactory cortex or archi- pallium. The fibers which go out from this center are. not well understood, but it is known that the neurites of cells in the epistriatum connect it with the striatum (tr. cortico-medialis). From this the tractus strio-thalamicus carries impulses to the thalamus. In the roof also is another fiber crossing which is said to be composed of true commissural fibers between the lateral olfactory nuclei. This commissure has no counterpart in other vertebrates, but an analogous commissure is seen in the medulla oblongata of some fishes. In some bony fishes the facial lobes are so largely developed as to fuse together over the ventricle and in the lobus impar so formed are found commissural fibers. So here, the lateral olfactory nuclei have arched over the ventricle and a large commissure is formed where they have fused together.
The scheme of the fiber tracts given in Figure 147 will make clear the general relations of the central olfactory apparatus. It should be noticed particularly that the tractus olfactohypothalamicus medialis curves down in front of the foramen of Monro.
In this figure there is shown also the root of the nervus terminalis which has its ending in the regio uncinata. The relations of the end nucleus of this nerve to the adjacent olfactory nuclei and the course of fibers arising in it have not yet been studied.
Fig. 148. A diagram of the fiber tracts in the forebrain of a bony fish. The figure follows in the main a figure by Goldstein, and is drawn on the same plan as Figs. 146 and 147.
The Forebrains of Ganoids and Bony Fishes
Can be treated together as they are much alike. The forebrain is elongated, the lamina terminalis is nearly horizontal, the lateral walls are thick but not high and the whole roof as far forward as the olfactory bulbs is membranous (Fig. 148). The median ventricle is large and spreads out wide under the membranous roof and at the cephalic end it bifurcates (foramina of Monro) into the ventricles of the olfactory bulbs. In the thick lateral walls are found nuclei corresponding to those in the selachian brain but much less well developed. The corpus striatum occupies the chief part of the basal and lateral wall and on its dorsal and internal surface is the epistriatum (Fig. 149). The olfactory lobe forms the whole thickness of the wall at the front part of the forebrain and covers the lateral surface of the striatum, but does not extend up into the roof. In the sturgeon a few nerve cells and fibers are found in the membranous roof which may be vestiges of the massive roof in selachians. In bony fishes the lateral olfactory nucleus is better developed than in ganoids and forms the nucleus thaeniae. The olfactory bulb lies forward as in selachians and is connected with the forebrain by a distinct olfactory tract. The olfactory tract in ganoids (Fig. 101) is distributed to all parts of the olfactory lobe and a part of it decussates in the cephalic part of the lamina terminalis to end in the epistriatum of the other side. In bony fishes the olfactory tract is in two bundles, a medial and a lateral. The lateral bundle ends in the lateral olfactory nucleus (nucleus thaeniae) and in some cases a part of it decussates in the anterior commissure. The medial bundle always decussates and is distributed to the lateral olfactory nucleus. In both ganoids and bony fishes there arises from the anterior part of the olfactory lobe a tractus olfacto-hypothalamicus median's, and from the lateral part of the lobe (nucleus thaeniae) a tractus olfactohypothalamicus lateralis. From the nucleus thaeniae of bony fishes and from the whole olfactory lobe and nucleus praeopticus of ganoids arise the fibers of the tractus olfacto-habenularis which decussate in part in the habenular commissure and end in the nucleus habenularis. The epistriatum in the sturgeon receives a large tract of ascending fibers from the hypothalamus, the tractus lobo-epistriaticus, which decussates in the anterior commissure. This tract may bring gustatory impulses to the epistriatum. The neurites of the epistriatum cells in the sturgeon find endings in the striatum; in bony fishes they are said to go in the tractus strio-thalamicus to the thalamus. The anterior commissure contains fibers from. the lateral olfactory nucleus to the epistriatum of the other side. From the nucleus praeopticus a tract goes back over the optic chiasma to the hypothalamus.
Fig. 149. A transverse section of the brain of the sturgeon behind the anterior commissure. Compare Fig. 99.
The scheme of fiber tracts given in Figure 148 shows that in essentials they agree with those of selachians. The olfactory lobe is smaller and does not extend up into the roof of the ventricle. The greater extent of the choroid plexus is due to the receding of the olfactory centers which in selachians help to form the roof. On this account there is no dorsal commissure but tertiary olfactory fibers cross in the anterior commissure and end in the epistriatum as in selachians. The primitive crossing of olfactory tract fibers which has not been seen in selachians is present in ganoids and bony fishes. The tracts to the hypothalamus and nucleus habenularis are essentially alike in all groups thus far described. The epistriatum of selachians is described as the place of ending of tertiary olfactory fibers only, while in ganoids it receives both secondary and tertiary olfactory tracts. Further investigation will probably show that the secondary olfactory fibers to the epistriatum are not absent in selachians. A prolonged discussion as to the nature of the pallium in bony fishes has been due especially to the elevation of the so-called pallium of selachians to the dignity of a true cortical center. Now that it is known that the pallium of selachians is a part of the olfactory lobe and that there is a close correspondence between the fiber tracts in selachians and bony fishes, it requires only the recognition of the slight development of the olfactory apparatus in bony fishes to explain the condition of the pallium. A greater or less part of the proper membranous roof has been displaced in selachians by the growing up, folding over and fusion of the lateral walls. In other fishes the corresponding part of the lateral wall is much smaller and the membranous roof is more extensive than in selachians. In some bony fishes (Fig. 151 C) there seems to be a sort of inversion of the lateral wall, so that the striatum and epistriatum bulge high up in the ventricle and the pallium passes over them and is attached to the lateral wall on the ventro-lateral aspect of the brain. A membranous roof over the median ventricle is universally present in the forebrain of vertebrates. In the more specialized selachians the greater part of it is displaced by the upgrowth from the lateral walls ; in bony fishes it is extraordinarily broad because of the reduction and receding of the lateral walls.
The nervus terminalis which is characteristic of selachian brains is found also in Amia, in Protopterus and in embryonic stages of Ceratodus, but nothing is known of its central connections.
The Forebrain of Amphibia
Presents two external peculiarities: first, the relatively great size and independence of the lateral lobes, and second, the f?ct that they extend forward far beyond the lamina terminalis. A glance at Figures 8 and 9 will show that the real difference between the amphibian and selachian brains in this regard is slight. In front of the lamina terminalis the lobes are separated by an open space known as the sagittal -fissure. In the frog this fissure is bridged across by the fusion of the two olfactory bulbs, but remains open behind the bulbs. The internal structure of the amphibian forebrain has been puzzling chiefly because its fiber tracts and commissures are difficult to compare with those of other vertebrates. An understanding of the gross relations in one of the lower tailed forms, such as the hellbender (Cryptobranchus) or mud-puppy (Necturus), will help to make the finer structure intelligible. The median ventricle is relatively short (Fig. 150). Its floor in front of the optic chiasma is formed by the nucleus praeopticus and farther forward by the lamina terminalis, which is very greatly thickened by two commissures crossing one above the other in nearly the same plane. In front of the commissures the lamina terminalis is nearly vertical in position and becomes continuous with the membranous roof. This is folded into the ventricle to form the choroid plexus. The recessus neuroporicus is recognizable in the frog a little above and in front of the commissure as in Petromyzon. The caudal part of the roof is produced dorsally into a more or less branched sac, the paraphysis. The median ventricle is connected by wide foramina of Monro with the lateral ventricles, which extend forward into the olfactory bulbs and backward into the rounded posterior part of the lateral lobes. For the purpose of describing centers and fiber tracts there may be distinguished in the lateral lobes a base or ventral wall, a lateral wall, a roof and a mesial wall. It will be convenient to anticipate the results of the following discussion and divide the mesial wall into a portion in front of the foramen of Monro called the septum (precommissural or paraterminal body), and a portion above the foramen of Monro which together with the adjacent dorsal and caudal parts of the hemisphere forms the hippocampus.
Fig. 150. A diagram of the fiber tracts in the forebrain of a tailed amphibian, Necturus. The representation of the commissure is based wholly upon Weigert sections of the brain of Necturus. In drawing the other tracts the descriptions of P. Ramon, Van Gehuchten and Bochenek for other amphibia have been consulted. The tractus olfacto-hypothalamicus lateralis is represented as joining the tractus stric-thalamicus from above and going off from it to the inferior lobe.
The fibers of the olfactory tract are distributed to the whole of the lateral wall and to the front part of the roof, base and septum and also to the nucleus praeopticus. A part of the olfactory tract decussates in the lower of the two commissures and goes to the lateral wall. As in selachians, then, the lateral and front walls of the hemispheres constitute the olfactory lobe. The base of the hemisphere is the corpus striatum. Fibers from the lateral wall pass back to the thalamus mingled with the tractus strio-thalamicus. They constitute a tractus olfacto-hypothalamicus lateralis, homologous with that in fishes. From the septum and part of the roof and base at the front end arise fibers which curve forward and downward and form a tract which runs backward near the mid-ventral line and enters the hypothalamus (tractus corticomedialis of P. Ramon in the frog). This corresponds to the tractus olfacto-hypothalamicus medialis in fishes. From the ventral and lateral part of the lateral wall, an area corresponding to the nucleus thaeniae in fishes, arises the tractus olfacto-habenularis which runs as in fishes. From the septum, which belongs to the olfactory lobe, fibers arise which run up and back and end in the caudal part of the mesial wall and roof, the tractus oljactorius septi of authors. This is a tertiary olfactory tract and corresponds to the tractus olfacto-corticalis in selachians, and should be given the same name.
The commissures are difficult to analyze and compare with those of other vertebrates. The lower commissure contains fibers of the tractus olfacto-hypothalamicus from the cephalic and lower half of the mesial wall (precommissural or paraterminal body, mesial olfactory nucleus), fibers of the tractus olfactorius which end in the recessus praeopticus, and fibers of the tractus striothalamicus. The lower commissure is placed in the base and lateral walls in very much the same position as the anterior commissure in fishes and it is usually called the anterior commissure proper. The upper commissure is very peculiar in its position and composition. It crosses from side to side in the lamina terminalis, causing a high ridge across the floor of the ventricle. At either side it rises up in the side walls just behind the foramen of Monro and bends forward over the foramen to be distributed to the upper half of the mesial wall and adjacent part of the roof. It also spreads back into the posterior part of the lateral lobe, into the part which is usually called the occipital pole. This name must not be taken to suggest any comparison with the occipital cortex of the human brain and would better be avoided.
Fig. 151. Simple diagrams to show the history of the epistriatum in fishes and amphibia. The epistriatum is represented by coarse dots adjoining the ventricle, and the figures indicate how it is involved in the changes of form of the forebrain and how there are developed from it the hippocampal formation and the epistriatum proper of the amphibian brain. A, Petromyzon; B, Acipenser; C, Teleost; D, Chimaera; E, Squalus acanthias; F, Necturus.
The upper commissure is usually described as a true commissure of the mesial and caudal regions just mentioned. It is, however, not so simple, for near its point of crossing it receives fibers from four other sources. First, fibers enter it from the lateral walls of the hemispheres; second, fibers from the medial olfactory nucleus; third, fibers from the tractus strio-thalamicus ; and fourth, fibers which come directly from the hypothalamus and enter the commissure above the tractus strio-thalamicus. The commissure also has a third place of distribution in the dorsal part of the brain, namely, in the lateral wall of the median ventricle anterior to the central gray of the thalamus. The composition and distribution of the commissure is shown in Figure 150. As it runs up behind the foramen of Monro it divides into two parts, the larger going into the medial wall of the hemisphere and the smaller turning into the lateral wall of the median ventricle. The latter bundle runs for a short distance ventral to and distinct from the tractus olfacto-habenularis and spreads out in the central gray cephalo-ventral to the nucleus habenulae. This region of central gray is clearly seen in Cryptobranchus, Necturus and in tadpoles of Amblystoma. It is a very compact body whose cells are arranged in rows next the ventricle and whose outer part is a fiber layer formed by the commissural bundle. In its structure, position and its relation to the tractus olfacto-habenularis, this body corresponds to the epistriatum of Petromyzon and to the caudal part of the epistriatum in Acipenser. As the cell layer is traced forward it curves through the foramen of Monro, forming the dorsal border of the foramen, and becomes continuous with the ventricular cell layer of the dorsal half of the mesial wall. Inasmuch as the commissure is distributed to both the lateral wall of the median ventricle and to the mesial wall of the lateral ventricle and as the ventricular cell layer is continuous throughout these regions, it is just to conclude that they form a structural and functional unit. The composition of the commissure gives further ground for regarding this whole region as homologous with the epistriatum in fishes, with which a part of it agrees in position. The epistriatum in fishes receives an ascending tract from the hypothalamus which decussates in the anterior commissure. Such a tract is not certainly found in the lower commissure in amphibia but is present in the upper commissure. These fibers presumably constitute the tractus lobo-epistriaticus and the area in which they end is the epistriatum. In selachians it has been noticed that the region called the epistriatum receives a tertiary olfactory tract from the mesial olfactory nucleus, the tractus olfacto-corticalis. In selachians and ganoids a part of this tract crosses to the opposite side. In amphibia the tractus olfactorius septi and the bundle which enters the upper commissure from the mesial olfactory nucleus are homologous with the tractus olfacto-corticalis, and the region to which the upper commissure is distributed must be considered as a part of the epistriatum. The part of the epistriatum which receives tertiary olfactory fibers is the olfactory cortex or hippocampus. A part of the epistriatum which does not receive such fibers covers the inner surface of the striatum and in all higher vertebrates is regarded as an integral part of that body.
Fig. 152. Sketches of transverse sections through (A) the caudal part of the epistriatum in amphibia and (B) the corresponding structure in Ornittiorhynchus. B from G. Elliot Smith. In A the tracts are drawn from the brain of Necturus and the cells in the epistriatum (E) are drawn schematically to show their disposition next the venticle and the continuity of the cell layer through the foramen of Monro with that of the hippocampal formation (Hipp.}, c, thetractuscortico-habenularis. On the right side the section is supposed to pass just behind the foramen of Monro.
In B: f.d., fascia dentata; f.m., foramen of Monro; hip., hippocampus; ne., neopallium; para., paraterminal body; plx.l., choroid plexus of the lateral ventricle ; rec.s., recessus' superior. The recessus superior extends dorsad between two vertical shaded lateral walls which correspond to the caudal part of the epistriatum in amphibia. This is erroneously called the paraphysis in mammals.
The amphibian hemispheres differ in two chief ways from those of selachians. First, the secondary olfactory centers are less massive, the wall is thinner and the ventricle larger, and the epistriatum is relatively larger. Second, there is a more complete folding of the lateral wall than in selachians. In selachians the lateral walls rose up, arched over and fused. In amphibia the lateral walls rose up and folded over without fusing with one another. The result is to form on each side a lateral hemisphere whose ventricle is a continuation of the primitive lateral or olfactory ventricle and whose dorso-mesial wall was primitively the dorsal part of the lateral wall of the median ventricle. The lateral ventricle of the hemisphere is not completely surrounded by nervous walls as is the primitive olfactory ventricle. In amphibia the mesial wall of each hemisphere is connected with its fellow over the median ventricle by a membranous roof, which is homologous with that in fishes. The diagrams in Figure 151 will show how the amphibian hemispheres have been formed by folding over of the lateral walls, and also the history of the epistriatum in fishes and amphibia. The epistriatum is in reality no more or less than the central gray matter of the forebrain in fishes. The dorsal part of this central gray in selachians receives olfactory fibers of the third order and in consequence becomes differentiated as the olfactory cortex. This region first becomes distinct in the amphibian brain as the result of the folding described. The caudal portion of the forebrain wall has not been folded into the lateral hemisphere and the epistriatum retains the position which it has in the caudal part of the forebrain in fishes. In higher forms this body becomes reduced but is still recognizable in mammals a short distance in front of the nucleus habenulae. Its nervous nature is evident in embryos but in the adult it has been confused with an entirely different structure under the name of paraphysis. In the accompanying Figure 152 the relations of the so-called paraphysis of a monotreme (Ornithorhynchus) are compared with those of the caudal part of the epistriatum in a tailed amphibian (Necturus). The structure which is properly known as the paraphysis is in lower vertebrates an outgrowth from the membranous roof of the forebrain hi front of the velum transversum (Figs. 36, 147 and 153).
Fi g- I 53- A sagittal section of the forebrain and interbrain of a chick embryo of 7.0 days. From Minot. F.B., forebrain; M.B., midbrain; chi., optic chiasma; ep., epiphysis; hy., hypophysis; Inf.g., infundibular gland (saccus vasculosus); A, paraphysial arch; Par., paraphysis; p.c., posterior commissure; V, velum.
The limits of the secondary and tertiary olfactory centers in amphibia are not clear but the dorso-medial and dorso-caudal parts of the hemisphere are at least chiefly tertiary. The internal structure of this region is more complex than in fishes. In addition to the pyramidal cells characteristic of the epistriatum of fishes there are many cells of type II and tangential cells present. It is clearly evident that this region constitutes an olfactory cortex and corresponds to the hippocampal formation in the mammalian brain. The commissure of this region must therefore be compared functionally with the hippocampal commissure. Anatomically, however, the commissure in amphibia is not the same as that in mammals. The upper commissure in amphibia lies beneath the median ventricle and behind the foramen of Monro and could not by any process of shifting be brought bodily into the position occupied by the hippocampal commissure in mammals. Moreover, the presence in this commissure of the ascending tract from the hypothalamus marks it as a part of the anterior commissure. In amphibia, then, the commissural fibers of the hippocampal formation run by way of the anterior commissure, in which they constitute a large bundle nearly separate from the rest of the commissure. The presence of the tract from the hypothalamus suggests that the olfactory cortex serves also as a gustatory cortex. There is finally to be mentioned a small tract from the dorsocaudal pole of the hemisphere which runs to the hypothalamus, taking a course over and behind the anterior commissure and separate from the tractus olfacto-hypothalamicus medialis, which runs beneath the anterior commissure. This small tract is to be compared with the jornix of the mammalian brain.
Fig. 154. Part of a transverse section of the cortex of a chameleon. After Cajal (Textura, etc.). A, superficial plexiform layer; B, layer of pyramids; C, deep plexiform layer; D, white substance; E, ependyma.
Fig. 155. Transverse section through the forebrain of Lacerta at the level of the anterior commissure. After Cajal (Textura, etc.).
No vestige of the nervus terminalis or its center has been recognized in the amphibian brain.
The Reptilian Brain
Differs from the amphibian chiefly in having larger hemispheres with a somewhat more complex structure in the olfactory cortex (Fig. 154), and in the position of the hippocampal commissure. The olfactory lobe occupies the anterior, lateral and ventral surfaces of the hemisphere, the base is the corpus striatum and the dorsal and medial walls constitute the hippocampus or olfactory cortex. The fiber tracts connecting the olfactory lobe with the cortex and connecting both with the hypothalamus and the nucleus habenulae seem to be identical with those in amphibia. The tract from the dorsocaudal pole to the hypothalamus is much larger than in amphibia. Overlying the striatum in the ventro-lateral wall of the hemisphere is a body called the epistriatum which represents the ventral, unspecialized portion of the epistriatum in fishes. The anterior commissure (Fig. 155) includes olfactory tract fibers, fibers from the lateral olfactory area to the epistriatum, and fibers of the tractus strio-thalamicus. The commissure of the hippocampal region crosses in the upper part of the lamina terminalis and passes in front of the foramen of Monro to reach the hippocampus. The position of the commissure is the same as that of the hippocampal commissure in monotremes and lower mammals. The tractus olfacto-habenularis receives fibers from the dorso-caudal pole of the hemisphere. These fibers constitute a tractus corticohabenularis. A commissure is found in lizards and some other reptiles which connects the dorso-caudal poles of the hemispheres directly, above the ventricle. This commissure runs in the velum transversum and does not correspond in position to any commissure found in other vertebrates. It is therefore called the commissura aberrans (Fig. 158). The position and extent of the several parts of the forebrain are shown in Figures 155, 156 and 157. The last figure does not show the whole extent of the hippocampus but shows the position of its mesial part relative to the lamina terminalis, the foramen of Monro, the commissures and the mesial olfactory nucleus (septum).
Fig. 156. A transverse section through the right lateral lobe of the forebrain of Lacerta. After Cajal (Textura, etc.). E, epistriatum; F, white substance of the hippocampal formation; S, striatum.
Fig. 157. A diagram of the mesial surface of the hemisphere of a reptile to show the extent of the hippocampus and related structures. The lamina terminalis is shaded with horizontal lines and in it are shown the anterior and hippocampal commissures. The hippocampal area is shaded with oblique lines, b, bulbus olfactorius; p, paraterminal or precommissural body (septum).
The Mammalian Forebrain
In monotremes and marsupials the relations of the olfactory centers are essentially as in amphibia and reptiles, although there is a higher development in internal structure. The olfactory bulbs and lobes are relatively small (Figs. 159, 160) and the rest of the hemisphere very large in proportion. Indeed, in monotremes and marsupials as in mammals, the lateral and dorsal walls of the hemispheres do not belong to the olfactory apparatus as in reptiles and lower forms, but constitute the somatic pallium. The olfactory cortex is now confined to a part of the mesial wall of the hemisphere. Its extent is best seen in a view of the mesial surface of the brain (Fig. 1 60). The dorsal part of the mesial wall belongs with the dorsal wall to the general cortex or neopallium whose functions are chiefly somatic. The region corresponding to the lateral olfactory nucleus of lower vertebrates is crowded down upon the ventral surface of the hemisphere and forms the pyrijorm lobe (Fig. 159). This is separated from the general cortex by the fissura rhinalis. The mesial olfactory nucleus is in the same position as in lower vertebrates, forming the tuberculum olfactorium and the lower portion of the mesial wall called the precommissural body (G. Elliot Smith). In the lamina terminalis is a very large anterior commissure and above it in the dorsal border of the lamina terminalis is a smaller hippocampal commissure. This commissure crosses in front of the median ventricle and enters the hippocampus above the foramen of Monro, and therefore corresponds in position to the hippocampal commissure of reptiles.
Fig. 158. A mesial sagittal section of the brain of an embryo of Sphenodon punctatum. From G. Elliot Smith (Aberrant Commissure, etc.). a. S., aqueduct of Sylvius; b.o., bulbus olfactorius; c.a., commissura aberrans; c.d., hippocampal commissure; c.h., habenular commissure; c.p., posterior commissure; c.v., anterior commissure; ce., cerebral hemisphere; hyp., hypophysis; l.t., lamina terminalis; opt., tractus opticus; par., paraphysis; p.o., olfactory peduncle; p.s., parietal stalk; plx.II.., lamina chorioidea; tub'., tuber cinereum; v.III., third ventricle.
Above the commissures is a slender prolongation of the median ventricle, forward beyond the foramen of Monro, which corresponds to the anterior portion of the median ventricle in amphibia (Fig. 150) and reptiles (Fig. 161), and ends in the homologue of the recessus neuroporicus of fishes. This part of the median ventricle has been called the recessus superior and in both reptiles and monotremes its walls contain nervous elements and correspond to the caudal part of the epistriatum in amphibia as explained above (cf. Figs. 152 and 161). A transverse section through the commissures of a monotreme (Fig. 162) shows the general cortex forming the lateral and dorsal wall of the lateral ventricles, while the strongly infolded mesial wall is the hippocampus. The infolding is marked by a groove which appears on the mesial surface of the hemisphere as the hippocampal fissure (Fig. 159). The lower border of the hippocampus shows a higher specialization of structure and is known as the fascia dentata. The lower wall of the brain is formed by the pyriform lobe and the corpus striatum through which runs the large anterior commissure.
Fig. 159. The mesial surface of the right half of the brain of Ornithorhynchus . From G. Elliot Smith.
Fig. 160. The mesial surface of the right cerebral hemisphere of a marsupial (Phascolarctos) . From G. Elliot Smith (Relation of Fornix, etc.) . a, extra ventricular alveus; d,d' , fascia dentata; /, fimbria; g, neopallium; /,/', lamina terminalisjo, olfactory bulb; o', olfactory peduncle; p, precommissural body; r, pyriform lobe;/, tuberculum olfactorium; v, ventral commissure (anterior commissure plus commissural fibers of the neopallium); w, hippocampal commissure; x, optic chiasma; y, thalamus.
Where the commissure crosses in the lamina terminalis the latter is thickened by gray matter which has invaded it from the adjacent mesial olfactory nucleus or paraterminal body (Fig. 163). In the upper part of the lamina terminalis the hippocampal commissure crosses in front of the foramen of Monro and covers the face of the hippocampus which bounds the lateral ventricle. Upon the upper surface of the hippocampal commissure in the middle line appears the small recessus superior. The hippocampal commissure is derived wholly from the hippocampal fold of the mesial wall of the hemisphere, while the dorsal and lateral wall or general cortex contributes fibers to the anterior commissure.
Fig. 161. Portion of a transverse section through the brain of a Monitor (Hydrosaurus). From G. Elliot Smith. In the figure to the left the line x-y, shows the plane of the section, alv., alveus; c.f., columna fornicis; fasc., fascic ulus marginalis; hip., hippocampus; para., paraterminal body; rec.s., recessus superior; c.d., hippocampal commissure; c.v., anterior commissure.
In sagittal section near the medial plane (Fig. 164) are to be seen the following tracts belonging to the olfactory apparatus. The tractus olfactorius (olfactory peduncle) enters the tuberculum olfactorium and the precommissural body and also- sends a part of its fibers up into that part of the hippocampal formation known as the fascia dentata. The latter fibers are a vestige of the olfactory tract fibers which run to the epistriatum in fishes. From the tuberculum olfactorium a large tract, not shown in the figure, goes up through the precommissural body to the hippocampus.
This corresponds to the tractus olfacto-corticalis of selachians, amphibia and reptiles (tractus olfactorius septi). A diffuse bundle of fine fibers arising in the precommissural body runs backward below the anterior commissure and is widely distributed in the hypothalamus (cf. p. 275). This tract is evidently identical with the tractus olfacto-hypothalamicus medialis of lower vertebrates. Fibers arising from the whole arch of the hippocampus (Fig. 160) collect toward the commissural region in the lamina terminalis.
Fig. 162. Transverse section through the cerebral hemispheres of Ornithorhynchus. From G. Elliot Smith, c.d., hippocampal commissure; c.v., anterior commissure plus neopallial fibers; f.d., fascia dentata; hip., hippocampus; I. p., lobus pyriformis; para., paraterminal body; rec.s., recessus superior.
Those from the caudal portion of the hippocampal arch run forward on its ventricular surface forming the fimbria. When the fimbria arrives at the lamina terminalis a part of its fibers go to form the hippocampal commissure (Fig. 164, w). The remainder of the fimbria fibers together with numerous fiber bundles from the whole anterior portion of the hippocampal formation collect into a large bundle close upon the upper surface of the anterior commissure and run backward through the hypothalamus to the corpus mammillare. This is the fornioc (columna fornicis) and its course is the same as that of the bundle called fornix in amphibia and reptiles. Just above and behind the anterior commissure the fornix column crosses the stria medullaris thalami which is coming up from the nucleus amygdalae in the caudal part of the ventral wall of the hemisphere. This region corresponds to the lateral olfactory nucleus of lower vertebrates and the stria is identical with the tractus olfacto-habenularis. The fibers which join this tract from the fornix columns constitute the tractus cortico-habenularis. Finally, a large tract (Fig. 164,5) runs up from the region just in front of the optic chiasma through the precommissural body and enters the fimbria to go to all parts of the hippocampus. These fibers probably come from the lateral olfactory area (pyriform lobe or nucleus amygdalae). They cor respond to the crossed tractus olfacto-corticalis of fishes and to the crossed sphenoido-hippocampal tract in higher mammals.
Fig. 163. Plan of cerebral hemispheres, lamina terminalis and optic thalami in horizontal section. From G. Elliot Smith, b.o., olfactory bulb; f.M., foramen of Monro; l.t., lamina terminalis; o.t., optic thalami; para., paraterminal body; v.L, lateral ventricle; v.III., third ventricle.
It was stated above that the region corresponding to the lateral olfactory nucleus of lower vertebrates is crowded down upon the ventral surface of the hemisphere to form the pyriform lobe. This lobe appears on the ventral aspect of the brain as a ridge which extends the whole length of the lower wall of the hemisphere (Fig. 165). As the olfactory tract comes from the bulb it divides into a mesial portion whose connections in the tuberculum olfactorium, precommissural body and hippocampus have just been described, and a lateral portion which enters the pyriform lobe. This lateral portion, the external root or external olfactory radiation of authors, is distributed through the whole length of the pyriform lobe and to the caudal portion of the ventral wall of the hemisphere from which arises the stria medullaris thalami mentioned above. In higher mammals the anterior part of this region is known as the nucleus amygdalae, the posterior part as the pyriform lobe or sphenoidal cortex and from it arises the taenia semicircularis to the hypothalamus (Fig. 166). This is homologous with the tractus olfacto-hypothalamicus lateralis in fishes. As indicated above a tract from this same region goes to the hippocampus by way of the precommissural body and the fimbria.
Fig. 164. Sagittal section of the commissural and precommissural regions of the right hemisphere of Ornithorhynchus. From G. Elliot Smith, am., tractus cortico-habenularis; b., nucleus habenulae; d, fascia dentata; g, neopallium; o f , olfactory peduncle; p., precommissural body; t., tuberculum olfactorium; v., anterior commissure plus neopallial fibers; w., hippocampal commissure; 2, column of fornix; 2', fibers of the same from the anterior end of the hippocampus passing beneath the anterior commissure; 3, fasciculus praecommissuralis, probably coming from the pyriform lobe; 4, tractus olfacto-hypothalamicus; 5, fasciculus marginalis, a part of the olfactory tract which goes to the hippocampus; 6, stria medullaris thalami.
Fig. 165. Ventral surface of the brain of Ornithorhynchus to show the position of the pyriform lobe. Outline after a figure by G. Elliot Smith, b., olfactory bulb; p., pyriform lobe; t. tuberculum olfactorium; g.c., general cortex or neopallium.
This general survey shows that there is no essential difference in the arrangement of the central olfactory apparatus in monotremes and in amphibia and reptiles. The large size of the anterior commissure is due to the fact that it contains large numbers of fibers connecting the lateral walls of the hemispheres. These lateral walls together with the dorsal part of the mesial wall of the hemisphere correspond to the similarly placed general cortex or somatic pallium in man. This region has been called neopallium to distinguish it from the olfactory cortex (archipallium) which is traceable continuously from selachian fishes to mammals. The neopallium has developed in the lateral and dorsal wall of the hemispheres, crowding between the mesial and lateral parts of the olfactory lobe and pushing the lateral part down upon the ventral surface. The commissural fibers of this neopallium in monotremes are to be compared functionally with the corpus callosum of man, but anatomically they are widely different. They run simply in the anterior commissure. In the brain of the simpler mammals little change from that of the monotreme and marsupial is to be noticed except in the disposition of the commissure of the neopallium. This change consists in the shifting of the commissural fibers of the neopallium from the anterior commissure to the hippocampal commissure. The fibers appear first in the front part of the hippocampal commissure, with the fibers of which they are mingled. As the neopallial fibers in this position increase in number the hippocampal fibers are displaced backward and the dorsal commissure comes to be formed of two parts, an anterior corpus callosum and a posterior hippocampal commissure or psalterium.
Fig. 1 66. Transverse section through the brain of the rat of four days at the level of the anterior commissure. From Cajal (Textura, etc.). A, columna fornicis; B, C and K tractus olfacto-hypothalamicus ("olfactory projection tract") ; D, lobus pyriformis; E, nucleus lentiformis of corpus striatum; F, tractus opticus; H, anterior commissure; /, cingulum; R, nucleus caudatus of corpus striatum; T, fasciculus longitudinalis superior.
Fig. 167. Diagram of the mesial surface of the hemisphere of (A) amonotreme and (B) that of a mammal to show the extent and relations of the hippocampus. Compare with Fig. 157. b, olfactory bulb; c.a., anterior commissure; c.c., corpus callosum; c.h., hippocampal commissure; p., paraterminal or precommissural body; pyr., pyriform lobe; t., tuberculum olfactorium; s., septum pellucidum.
The further development of the mammalian forebrain consists chiefly hi the enlargement and increasing complexity of the neopallium, the enlargement of the corpus callosum and the shifting of parts consequent on these changes. The enlargement of the neopallium and corpus callosum results in the degeneration of the hippocampus through the greater part of its length. The facts of chief importance are to be seen in the mesial aspect of the hemisphere of various mammals or in sagittal sections near the median plane. In the monotreme and marsupial (Fig. 167) the hippocampus begins just above and behind the olfactory peduncle, where it forms the boundary between the general cortex (neopallium) and the precommissural body, and extends back above the foramen of Monro and forms the mesial margin of the roof of the hemisphere. Some distance from its anterior end the hippocampus crosses the dorsal border of the lamina terminalis, to which it is attached by reason of the fact that its commissure crosses through the border of the lamina. Here also, just above and in front of the commissure the lamina terminalis joins the membranous roof of the median ventricle, the angle marking the position of the recessus neuroporicus in the embryo. In the marsupial and simplest mammalian brains the hippocampal commissure takes on the form of a compact lamina and the corpus callosum extends forward from it, so that the two form an inverted V-shaped figure whose caudal descending limb is the hippocampal commissure. The two commissures cause a thickening of the lamina terminalis as indicated in the accompanying diagram (Fig. 168, C).
The fibers of the corpus callosum in assuming this position have run transversely through the hippocampal formation and have mingled with the hippocampal ends of all the longitudinal fiber tracts which have been described above as forming the fornix system. It is very important that this intermingling of the transverse fibers of the corpus callosum with the longitudinal fibers which connect the hippocampus with the olfactory lobe, the base of the brain and the hypothalamus should be held in mind. The effect of the corpus callosum piercing the hippocampus is to cause it to degenerate in its middle part, adjacent to the lamina terminalis. In more highly developed (mammalian) brains the hippocampus from this point forward degenerates to a mere rudiment, the position of the original hippocampus being occupied by the enlarged corpus callosum. The rudiment of the hippocampus remains on the dorsal surface of the corpus callosum and extends forward from it along the line of demarcation between the precommissural body and the neopallium. The fibers of the several tracts of the fornix system which in monotremes entered the hippocampus anterior to the lamina terminalis are preserved in small numbers and still enter the rudimentary hippocampus, either running around the front edge (genu) of the corpus callosum or piercing it (perforating fibers of authors, fornix longus of Forel). These perforating fibers maintain the same position and course which they have in all vertebrates, the corpus callosum is a new structure which has run transversely through and mingled with them, as explained above.
Fig. 1 68. Schemes to explain the expansion of the corpus callosum and the formation of the septum pellucidum. After G.Elliot Smith. A, reptile; B,monotreme; C, marsupial; D, bat; E, higher mammal, c.a., anterior commissure; c.c., corpus callosum; c.h., hippocampal commissure; fi., fimbria; hipp., hippocampus; ind., indusium; p., precommissural body.
The growth of the corpus callosum not only reduces the hippocampus to a rudiment but also changes its position. As the neopallium enlarges and spreads backward the callosum spreads in the same direction, pushing back part of the hippocampus before it and stretching the rudimentary hippocampus which lies on its dorsal surface. The corpus callosum is formed in the' lamina terminalis and its caudal border is mingled with the hippdcampal commissure. As the callosum tends to grow broader antero-posteriorly its two borders are confined. In order to widen it must either stretch the lamina terminalis backward or it must bend and fold upon itself. It does both. In the simplest mammals (bats) no change is seen from the marsupial condition, but in such animals as the rabbit and hedgehog the corpus callosum has arched upward in the form of an inverted U, stretching the rudimentary hippocampus (Fig. 168). In higher mammals such as the cat, and in man the callosum has grown much larger and has pushed back over the hippocampal commissure. At the same time the U-shaped bend in the callosum has become more and more sharp until the two limbs have met and in man it appears on superficial examination that the caudal border of the callosum is merely thickened. In fact, this portion, known as the splenium, is formed by an actual folding of the callosum due to its growth in width while its borders remained fixed. The real border of the callosum is beneath its body, some distance in front of the splenial border, adjoining the psalterium (Fig. 169, 1"). Not only does the folded callosum extend back over the hippocampal commissure, but it stretches that part of the lamina terminalis containing that commissure backward far from its original position. This stretching of the lamina terminalis is evident upon comparing a sagittal section of the bat or rabbit brain with that of the cat or man. It is shown diagrammatically in Figure 168. A further effect is to stretch the mesial walls of the hemisphere at the same time with the lamina terminalis. As the anterior, hippocampal and callosal commissures form in the lamina terminalis the lamina becomes thickened by the invasion of gray substance from the adjacent precommissural body. This gray substance forms a bed for the commissural fibers. As the callosum expands and bulges and folds upward and backward it stretches the commissure-bed and also the closely related precommissural body, until the upper part of that body is drawn out into a thin membrane. As this occurs in the mesial walls of both hemispheres two thin membranes are formed facing each other and filling in the somewhat triangular space between the two limbs of the callosum. These membranes contain a few nerve cells and fibers of the fornix system and constitute the two leaves of the septum pellucidum. The space between the two leaves of the septum pellucidum is merely a part of the great sagittal fissure of the brain which by this process becomes roofed in by the corpus callosum and is called the "fifth ventricle". While these changes in the commissures are taking place the part of the hippocampus which retains its full development is pushed back until it no longer touches the lamina terminalis in higher mammals (Fig. 169).
Fig. 169. Scheme of cerebral commissures and margin of the cortex in the human brain. From G. Elliot Smith, a'. a", extra ventricular alveus; c,c', corpus callosum; d, fascia dentata; /,/',/", fimbria; h',h",h f ", reduced hippocampus; 1,1', I", lamina terminalis; 0,0' ,o", olfactory bulb and peduncle; p, precommissural body; p', septum pellucidum; r, pyriform lobe; s, splenium of corpus callosum; v, anterior commissure; x, optic chiasma.
While the expansion of the neopallium and its commissure has been so profoundly affecting the form of the hemisphere and the relations of the archipallium, the hippocampal formation itself has undergone changes of form and become more complex in structure. In monotremes and marsupials the hippocampus occupies the mesial wall of the hemisphere and is slightly infolded into the lateral ventricle, the line of infolding being marked by the hippocampal fissure. Along the lower border of the hippocampus the cells multiply and proliferate from the ventricular layer to form the fascia dentata. As the hippocampus folds more strongly and the fissure deepens the thick fascia dentata which forms the lower limb of the fold retains its position on the surface and the upper limb, or hippocampus proper, is drawn deeper in and wrapped around the fascia dentata (Fig. 170) until the greater part of it is submerged within the hippocampal fissure. In most mammals the in- rolling goes so far that a larger or smaller part of the ventricular surface of the hippocampus is brought out upon the external surface of the brain where it is exposed except for a thin layer of fibers belonging to the fimbria. This inverted portion is the only part of the hippocampus, except the fascia dentata, which is exposed to view in the human brain. Of the two parts of the hippocampal formation the fascia dentata seems to serve wholly for the reception of olfactory impulses and to send its fibers into the hippocampus proper, while the hippocampus receives fibers from olfactory and other centers and sends out commissural fibers and fibers df projection to the corpus mammillare and nucleus habenulae.
This long and complex history of the evolution of the cerebral hemispheres may be summarized by giving a brief review of the chief parts of the hemisphere of a higher mammal or man, with an indication of their homologues in lower vertebrates. The base of the hemisphere is formed chiefly by the corpus striatum which includes the caudate and lentiform nuclei and is traversed by ascending and descending fiber tracts to the general cortex and hippocampus. In addition to these, many fibers come up to the corpus striatum from the lower parts of the brain and end in it, and a smaller number from the pyramidal cells of the general cortex also end here. Fibers arising from the cells of the striatum end in the centers of the thalamus, forming a tractus strio-thalamicus. The nucleus caudatus, from its position next to the ventricle and from the fact that a large part of its fibers end in the nucleus lentiformis, strongly reminds one of the unspecialized epistriatum of fishes.
Fig. 170. Scheme of the structure and connections of the hippocampus. From Cajal (Textura, etc.). A, ganglion of the occipital pole; B, subiculum; C, hippocampus; D, fascia dentata; E, fimbria: F, cingulum; G. crossed sphenoidohippocampal tract; H, corpus callosum; a, neurites entering the cingulum; b, fibers of the cingulum ending in the occipital pole; c, perforating fibers of the sphenoido-hippocampal tract; g, cell of the subiculum.
The ventral surface of the striatum is covered by the nucleus amygdalae which is continuous with the pyriform lobe or sphenoidal cortex. These areas together correspond to the lateral olfactory area of fishes, amphibia and reptiles. From this region arise three tracts; first, the stria medullaris or taenia thalami, to the nucleus habenulae (tractus olfacto-habenularis) ; second, the thaenia semicircularis, to the hypothalamus (tractus olfacto-hypothalamicus lateralis) ; third, a tract to the hippocampus by way of of the precommissural body (gyrus subcallosus) and the fimbria (tractus olfacto-corticalis).
The inner surface of the corpus striatum forms the floor of the lateral ventricle, the anterior horn of which extends in front of the striatum into the frontal lobe of the hemisphere and reaches into the olfactory bulb, except where it becomes obliterated in the adult as in man. This is homologous with the primitive lateral or olfactory ventricle in lower vertebrates. The ventral and mesial wall of the anterior horn in front of the lamina terminalis is formed by the anterior and mesial part of the secondary olfactory center, homologous with the mesial olfactory nucleus in fishes. In man this region includes the tuberculum olfactorium and the small region in front of the lamina terminalis known as the gyrus subcallosus, better called the precommissural body. In the lamina terminalis in front of the median ventricle is a small anterior commissure connecting the corpora striata. This is largely made up of olfactory tract fibers coming from the olfactory bulb. The lamina terminalis is continued upward and greatly stretched backward to join the splenium of the corpus callosum. In this stretched portion of the lamina terminalis runs a thin band of transverse fibers constituting a commissure of the hippocampi, the psalterium. The corpus callosum is a thick lamina of transverse fibers belonging to the neopallium, slightly arched. The mesial wall beneath it is formed of the stretched and thinned dorsal portion of the precommissural body and is known as the septum pellucidum.
The floor of the temporal horn of the lateral ventricle is formed by the hippocampus which approaches the under side of the splenium of the callosum. The hippocampus does not end here but bends backward and curves over the splenial border of the callosum to run forward upon the dorsal surface of the latter. The portion of the hippocampus in this position consists of paired rudiments of the hippocampal fold (indusium falsum) in which runs a strand of fibers belonging to the fornix system, the striae Lancisi^ and a median film of gray matter connecting the hippocampal rudiments (indusium verum). This rudimentary hippocampus runs the whole length of the callosum in man, and in some mammals continues forward to the olfactory peduncles, forming the boundary line between the praecommissural area and the neopallium in the original position of the hippocampus in monotremes and marsupials. From the chief part of the hippocampus in the temporal region fibers run forward over its surface forming the fimbria. When the hippocampus bends back to gain the upper surface of the callosum the fimbria continues forward in the lamina terminalis, and is usually termed the body of the fornix. The two bodies converge forward and the triangular space between them is bridged by the exchange of fibers which form the psalterium. This is a true commissure of the hippocampus and is homologous with the hippocampal commissure of reptiles and monotremes. In the lamina terminalis the bodies of the fornix bend downward in front of the foramina of Monro and then backward to become the pillars of the fornix. From the rudimentary hippocampus above the callosum similar fibers run in the striae Lancisii and eventually break through the callosum or run around its anterior border and through the septum to join the pillars of the fornix. The latter then run into the thalamus and end in the corpus mammillare. Fibers of the olfactory tracts and fibers from the tuberculum olfactorium and precommissural body enter the hippocampus by way of the septum and fimbria or as fibers perforating the callosum. Fibers running by the same routes come to the hippocampus from the pyriform lobe and nucleus amygdalae. The hippocampus also receives or sends out fibers through the corpus striatum. It is possible that these latter include the equivalent of the tractus lobo-epistriaticus of fishes and that they bring up gustatory impulses from the hypothalamus. It is especially interesting in this connection to note that gustatory sensation is thought by Flechsig to be localized in the hippocampus or the area immediately adjoining it.
A remarkable Constance and similarity of structure is seen in the olfactory central apparatus throughout the classes of vertebrates. Speaking broadly, the primary (bulb) and secondary centers (lobe) and cortex with their respective tracts are already formed hi selachians upon a plan which is retained in all higher vertebrates. A relative increase hi the tertiary (cortical) centers and an increasing complexity of structure in these centers is the chief difference between higher and lower vertebrates. In fishes, amphibia and reptiles, as far as known, the whole forebrain with the exception of the center for the nervus terminalis in those forms which possess it, is devoted to the olfactory functions. The cortical center also serves the gustatory system. Apparently suddenly in monotremes appears a large lateral and dorsal cortex which is devoted to somatic functions. This is the neopallium whose structure and functions will be considered in the next chapter. Its position and the relations of its commissure show that it began its history in a dorso-lateral position at the anterior end of the forebrain between the mesial and lateral olfactory nuclei, and that it spread back from this point, pushing the two olfactory areas down upon the ventral surface and crowding the olfactory cortex to the extreme dorso-mesial margin. The neopallial commissure at first ran in the primitive forebrain commissure (anterior commissure) and later followed the movements of the hippocampal commissure upward to the dorsal edge of the lamina terminalis. The longitudinal tracts of the neopallium pass up and down through the corpus striatum lateral to and independent from the olfactory tracts and to quite different destinations (Fig. 166). It is impossible to believe in the face of all that we know of the evolution of structure in the central nervous system that the neopallium has appeared spontaneously as a new formation in the brain of primitive mammals. It has arisen by the greater development of some structure previously existing in the vertebrate brain. That it should have arisen by modification of some part of the olfactory centers is inconsistent with all that has gone before in the present volume. The neopallium may be wholly absent from the brains of existing members of the classes below mammals but those brains have by no means been studied exhaustively enough to warrant such a statement. Indications of the existence of a homologue of the neopallium are to be looked for in the form of tracts connecting somatic sensory centers with centers in the forebrain isolated from the olfactory apparatus. A tract is certainly present in fishes running from the tectum opticum to the forebrain, but the center has not been isolated, perhaps because of its small size in all submammalian classes. The existence of a cutaneous nerve connected with the forebrain in many fishes (nervus terminalis) gives encouragement to the expectation that a forerunner of the neopallium may be found in lower vertebrates in the form of a segment of the somatic sensory column in the forebrain. It is to be expected that further study of amphibia and reptiles from this point of view will reveal the rudiment of the neopallium, whose rapid development in primitive mammals led to the dominance of this class of vertebrates.
Demonstration or Laboratory Work
- Upon careful dissections of the forebrain of a selachian, a bony fish, an amphibian and a mammal verify the general morphological relations described in this chapter.
- Upon Golgi or Weigert sections study the forebrain tracts in the selachian and amphibian brain and if possible in the brain of a small rodent, bat or mole.
- If material is available, study with the aid of Smith's descriptions the history of the hippocampus and the fornix system by means of dissections and Weigert sections of the brain of the opossum, bat, mole, guinea pig, rat, rabbit, and cat.
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