FIG. 75.-A deeper dissection of the lateral ventricle, and of the velum interpositum. a, under surface of corpus callosum, turned back; b, b, posterior pillars of the fornix, turned back; c, c, anterior pillars of the fornix; d, velum interpositum and veins of Galen; e, fifth ventricle; f, f. corpus striatum; 9. g, tænia semicircularis; h, h, optic thalamus; &, choroid plexus; 1, tænia hippocampi; m, hippocampus major in descending cornu; n, hippocampus minor; o, eminentia collateralis.

oy a transverse incision, and the posterior half of this structure be turned back, the body of the fornix on which the corpus callosum rests is exposed. If the anterior half of the corpus callosum be now turned forward, the grey partition, or septum lucidum, between the two lateral ventricles is exposed. This septum fits into the interval between the under surface of the corpus callosum and the upper surface of the anterior part of the fornix. It consists of two layers of grey matter, between which is a narrow vertical mesial space, the fifth ventricle. If the septum be now removed, the anterior part of the fornix is brought into view.

The fornix or arch is an arch-shaped band of nerve fibres extending in the antero-posterior direction. Its anterior end forms the anterior piers or pillars of the arch, its posterior end the posterior piers or pillars, whilst the intermediate body of the fornix forms the summit or crown of the arch. It consists of two lateral halves, one belonging to each hemisphere. At the summit of the arch the two lateral halves are conjoined to form the body; but in front of the body the two halves separate from each other, and form two anterior pillars, which descend in front of the third ventricle to the base of the cerebrum, where they form the corpora albicantia, and then enter the substance of the optic thalamus. Behind the body the two halves diverge much more from each other, and form the posterior pillars; each of which curves downwards and outwards into the descending cornu of the ventricle, and, under the

The velum interpositum is an expanded fold of pia mater, which passes into the interior of the hemispheres through the great transverse fissure. It is triangular in shape; its base is in a line with the posterior end of the corpus callosum, where it is continuous with the external pia mater; its lateral margins are fringed by the choroid plexuses, which are seen in the bodies and descending cornua of the lateral ventricles, where they are invested by the endothelial lining of those cavities. Its apex, where the two choroid plexuses blend with each other, lies just behind the anterior pillars of the fornix. The interval between the apex and these pillars is the aperture of communication between the two lateral ventricles and the third, already referred to as the foramen of Monro. The choroid plexuses contain the small choroidal arteries, which supply the corpora striata, optic thalami, and corpora quadrigemina; and the blood from these bodies is returned by small veins, which join to form the veins of Galen (Fig. 75). These veins pass along the centre of the velum, and, as is shown in Fig. 63, open into the straight sinus. If the velum interpositum be now carefully raised from before backwards, the optic thalami, third ventricle, pineal gland, and corpora quadrigemina are exposed.

The optic thalamus is a large, somewhat ovoid body situated behind the corpus striatum, and above the crus cerebri. Its upper surface is partly seen in the floor of the body of the lateral ventricle, but is for the most part covered by the fornix and velum interpositum. Its posteroinferior surface forms the roof of the descending cornu of the ventricle, whilst its inner surface forms the side wall of the third ventricle. At its outer and posterior part are two slight elevations, placed one on each side of the optic tract, and named respectively corpus geniculatum internum and externum.

The third ventricle is a cavity situated in the mesial plane between the two optic thalami. Its roof is formed by the velum interpositum and body of fornix; its floor, by the pons Tarini, corpora albicantia, tuber cinereum, infundibulum, and optic commissure; its anterior boundary, by the anterior pillars of the fornix, anterior commissure, and lamina cinerea; its posterior boundary, by the corpora quadrigemina and posterior commissure. The cavity of this ventricle is of small size in the living head, for the inner surfaces of the two thalami are connected together by intermediate grey matter, named the middle or soft commissure; but in taking the brain out of the cranial cavity this commissure is usually more or less torn through, and the cavity is consequently enlarged. Immediately in front of the corpora quadrigemina, the white fibres of the posterior commissure pass across between the two optic thalami. If the anterior pillars of the fornix be separated from each other, the white fibres of the anterior commissure may be seen entering the two corpora striata.

The pineal body is a reddish cone-shaped body, enveloped by the velum interpositum, and situated upon the more anterior pair of the corpora quadrigemina. From its broad anterior end two white bands, the peduncles of the pineal body, pass forwards, one on the inner side of each optio thalamus. Each peduncle joins, along with the tænia semicircularis, the anterior pillar of the fornix of its own side. In its structure this body consists of a vascular stroma of connective tissue, in the meshes of which lymphoid cells are contained. Branched corpuscles are also found not unlike nerve cells. Amylaceous and gritty

calcareous particles, constituting the brain sand, are also found in it. Usually it is hollowed out into two or more small cavities. The function of the pineal body is not understood, but both it and the pituitary body, which possess a certain structural correspondence, are usually referred to the type of the ductless glands.

The corpora quadrigemina or optic lobes are situated behind and between the two optic thalami, and rest upon the posterior surface of the crura cerebri. The division into two lateral halves is marked by a shallow longitudinal fissure, and the subdivision of each half into an anterior and a posterior eminence, by a shallow transverse fissure. The anterior pair of eminences are called nates; the posterior, testes. From each testis a strong white band, the superior peduncle of the cerebellum, passes backwards to the cerebellum, and stretching between the pair of peduncles is the valve of Vieussens or anterior medullary velum. The corpora quadrigemina are tunnelled in the antero-posterior direction by the aqueduct of Sylvius, which opens anteriorly into the third ventricle immediately below the posterior commissure, and posteriorly into the fourth ventricle under cover of the valve of Vieussens. It is lined by a cylindrical ciliated endothelium.

INTERNAL STRUCTURE OF THE CEREBRUM.-The cerebrum is composed both of grey and white matter, the general relations of these two forms of nerve matter to each other may be seen by making sections through the cerebrum. The determination, however, of their minute structure, and of the relations and connections of the nerve fibres to the nerve cells is, owing to the delicacy of the organ, one of the most difficult departments of anatomical study. Several anatomists have endeavoured to trace out the course of the nerve fibres in the organ, and though our knowledge is by no means complete, yet many important facts have undoubtedly been ascertained. These facts have been summarised, and numerous valuable additions made to them by Meynert in a recent elaborate memoir, which has been frequently consulted and made use of in writing the following description.

The Grey Matter of the cerebrum is disposed in three great groups: a, The grey matter of the cortex of the hemispheres; 6, the grey matter of the great ganglia of the base of the cerebrum; c, the central grey matter which forms the wall of the cerebral end of the cerebro-spinal tube.

a, The grey matter of the cortex of the hemisphere forms the superficial part of the convolutions, and is known as the great hemispherical ganglion, but in some localities, ast at the loci perforati antici and the septum lucidum, it has received distinctive names. When a convolution is divided vertically the grey matter is seen to be confined to the surface and to enclose a white core. The grey matter presents a laminated appearance, and as a rule consists of five or six layers, which are composed of the characteristic pyramidal nerve cells of the cortex of the cerebrum, of nerve fibres, of matrix or neuroglia, and of blood-vessels. The most superficial layer consists of neuroglia, in which nerve fibres extend parallel to the surface of the convolutions. In the deeper layers are found the pyramidal nerve cells, which lie with their long axes vertical to the surface of the convolutions, and which contain angular nuclei. From the observations of Lockhart Clarke, Arndt, Cleland, and Meynert, there can be no doubt that the pyramidal nerve cells vary in relative size and in numbers in the different layers of the grey cortex, and that the largest sized pyramidal cells lie in the third and fourth layers. L. Clarke stated that the cells of all the layers of the posterior or occipital lobe were small and of nearly uniform size, whilst in the convolutions anterior to it numerous cells of a much larger kind were found; but though it is undoubtedly true that large pyramidal cells are found in the frontal lobe in

considerable numbers, and that the greater number of the cells of the occipital lobe are small and nearly uniform in size, there is no difficulty in recognising in the occipital lobe a small proportion of cells, quite equal in magnitude to the largest cells of the frontal lobe, interspersed amongst the smaller pyramidal cells. The nerve fibres which ascend into the grey matter from the white core of the convolution radiate into its several layers, and are apparently continuous with the basal axis-cylinder processes of the nerve cells. According to Cleland, the elongated apices of the cells, which are directed to the surface of the convolution, are continuous with the nerve fibres situated in the superficial layer of horizontal fibres. Immediately subjacent to the large pyramidal cells numerous small, irregularly shaped nerve corpuscles, like those of the internal granule layer of the retina, form the so-called granule layer of the grey matter. Fusiform cells, which give off lateral processes, are found in the deepest layer of the grey matter, and form the claustral layer of Meynert. Gerlach has described here, as in the spinal cord, a network of extremely minute nerve fibres, with which the branched lateral processes of the nerve cells are apparently continuous. The neuroglia contains multitudes of small rounded corpuscles. In it also are found small stellate cells, provided with minute branched processes, which cells, as Meynert states, are so pellucid, that in the healthy brain they seem to be only free nuclei ; it is difficult to say whether these cells belong to the neuroglia, or are nerve cell elements. The grey cortex of the cerebrum is much more vascular than the white matter. The arteries derived from the pia mater pass vertically into it, and end in a close polygonal network of capillaries; but it is also traversed by the arteries, which terminate in the capillary network of supply for the white matter.

In the grey matter of the cortex of the occipital lobe eight layers have been described by Clarke and Meynert. The increase in number is due to the intercalation of two additional granule layers, which coalesce and form a distinct white band in the grey matter, owing, as Meynert states, to the absence of pigment in the cells of the granule layers.

The grey matter of the cortex of the island of Reil and of the convolutions bounding the Sylvian fissure contains a very large proportion of fusiform cells. They form the chief constituent of the grey claustrum, situated deeper than the grey matter of the island, and separated from the outer part of the corpus striatum by a thin layer of white matter. Fusiform cells also occur abundantly in the nucleus amygdala, a grey mass situated below the corpus striatum, which in some sections seems as if isolated, but in reality is continuous with the grey matter of the inferior temporosphenoidal convolution.

The grey matter of the cortex of the gyrus hippocampi and of the hippocampus major is apparently destitute of both the granule and claustral layers of cells. Its superficial layer has been named the nuclear lamina, and contains small and scattered nerve corpuscles. Next this lamina lies the striatum reticulare, in which the apices of the numerous pyramidal cells of the third layer branch and again unite to form a delicate network. Deeper than the pyramidal cells is a thick layer of so-called “granules,” which A. B. Stirling recognised some years ago as like the granules of the rust coloured layer of the cerebellum; like them they consist of a well-defined nucleus invested by delicate branched protoplasm. The grey matter of the two layers of the septum lucidum, though included between the corpus callosum and fornix, is yet in the same plane as the grey matter of the cortex of the inner surface of the hemispheres, but is cut off from it by the development of the transverse fibres of the corpus callosum, The grey matter of the locus perforatus anticus contains

b, The great ganglia of the base of the cerebrum are the corpora striata, the optic thalami, the corpora geniculata, the corpora quadrigemina, and the locus niger in each crus

cerebri.

The corpus striatum cerebri consists of two masses of grey matter separated from each other by numerous striæ of white fibres, which ascend from below upwards through its substance. The upper mass of grey matter projects into the lateral ventricle, and is called the intra-ventricular portion or nucleus caudatus. The lower extra-ventricular portion or nucleus lenticularis forms the outer and lower part of the corpus striatum, and is separated by the claustrum from the island of Reil. Multipolar nerve cells are found in both the caudate and lenticular masses, and in the latter cells of large size have been seen. The optic thalamus forms an almost continuous mass of grey matter traversed by nerve fibres, which are not, however, collected into definite striæ. The nerve cells in the grey matter are both multipolar and fusiform. The external corpus geniculatum consists of alternate layers of grey and white matter, due to the zig-zag folding of the grey matter; the nerve cells are multipolar, and contain pigment. In the internal corpus geniculatum the cells are smaller in size and fusiform. The grey matter of the corpora quadrigemina consists of two distinct masses. One, the zonular layer, lies near the surface, and contains small multipolar nerve cells; the other, the Sylvian or central layer, lies at the sides of the Sylvian fissure and belongs to the grey matter of the wall of the cerebro-spinal tube, and serves as a centre of origin for the roots of both the 3d and 4th cranial nerves. The grey matter of the crus cerebri occupies the centre of the cerebral peduncle. Its cells are multipolar, and contain dark brown or black pigment, so that the name locus niger is applied to this collection of nerve cells.

c, The central grey matter of the cerebrum is in series with the grey matter of the floor of the 4th ventricle and the grey matter of the spinal cord. It is situated around the Sylvian aqueduct, and at the sides and floor of the third ventricle, which form the cerebral portion of the cerebrospinal tube. That which is situated in relation with the aqueduct of Sylvius forms the Sylvian or central layer just described in the corpora quadrigemina. That which lies in relation to the third ventricle forms the middle or soft commissure, and the well-defined grey layer which covers the inner wall of each optic thalamus; also the grey masses situated at the base of the brain between and in front of the crura cerebri, viz., the pons Tarini, tuber cinereum, lamina cinerea, infundibulum, and the grey matter of the pituitary body. By some anatomists the grey matter of the pineal body is referred to the same category, but Arnold has pointed out that it is separated by its peduncle from the soft commissure; and Meynert is disposed to regard it as a ganglion of origin of the tegmentum. Both the pituitary and pineal bodies contain, besides the nervous matter, structures of the type of the glands without ducts.

The White Matter of the cerebrum consists of tracts or fasciculi of nerve fibres, of which-a, some connect the cerebrum with the lower divisions of the encephalon; b, others connect the two hemispheres together; c, others connect different structures in the same hemisphere; d, others serve as roots of origin for the more anterior encephalic nerves.

a, The tracts of fibres which connect the cerebrum with the lower divisions of the encephalon are called peduncular fibres. The largest of these peduncles are the two crura cerebri or cerebral peduncles. Continuous below with the longitudinal fibres of the pons they ascend into the optic thalami and corpora striata, and their fibres are named the peduncular fibres. From the corpora striata and optic

the hemisphere and form the corona radiata. To some extent the fibres of the corona are directly continuous with those of the cerebral peduncles, but there can be no doubt that a large portion of the peduncular fibres terminate in the grey matter of the ganglia of the base of the cerebrum, and that a still larger number arise from their nerve cells to aid in the formation of the corona radiata. The direct continuity, therefore, of many of the peduncular fibres with those of the corona is broken or interrupted by the interposition of the cerebral ganglia, which Meynert has named ganglia of interruption. The peduncular fibres and those White of the corona constitute the cerebral portion of the projection matter. system of fibres of Meynert, a term devised to express that they conduct upwards to the grey cortex of the hemispheres sensory impulses derived from the external world, the image of which is projected upon the cortex. But it should also not be forgotten that many of the fibres of this system conduct motor impulses downwards to be propagated along the motor cranial and spinal nerves. The peduncular fibres of the crura cerebri are arranged in two groups, named respectively crusta and tegmentum, which are separated from each other by the nerve cells of the locus niger. The crusta forms the superficial or anterior part of the crus. Its fibres are in greater part continuous with the longitudinal fibres of the pons derived from the anterior pyramids of the medulla; but it receives additional fibres from the grey matter of the locus niger, and from the cells of the Sylvian layer in the corpora quadrigemina. Some of the fibres of the crusta pass directly upwards as radiating fibres to the grey cortex of the occipital and temporal lobes, but the larger number terminate in the nucleus caudatus and nucleus lenticularis of the corpus striatum. From these nuclei a great mass of fibres radiates into the cortex of the fronto-parietal lobes, more especially the frontal, but a few also, bearing the special name of stria cornea, pass to the grey matter of the apex of the temporal lobe; fibres also enter the convolutions of the insula. In addition to the radiating fibres, the grey matter of the corpus striatum gives origin to fibres of the middle root of the olfactory peduncle, and to connecting fibres with the grey matter of the septum lucidum. The tegmentum forms the posterior or deeper part of the crus cerebri. Its fibres are continuous with the longitudinal fibres of the pons derived from the olivary fasciculi, fasciculi teretes, and posterior pyramids of the medulla. A few of the fibres of the tegmentum enter the corpora quadrigemina and corpora geniculata, but the great majority enter the optic thalami, in the grey matter of which many evidently terminate, though some may pass through into the cortex of the hemispheres as fibres of the corona radiata. But the grey matter of the thalamus gives origin to numerous radiating fibres: those which arise in its posterior part radiate into the occipital and temporal lobes, whilst those proceeding out of its anterior part radiate into the frontal, parietal, and temporal lobes, and the insula. In the optic thalamus the fornix arises. Its fibres emerge from the under surface of the thalamus, form the corpus albicans, and then pass backwards as the upper boundary of the great transverse fissure to end as the tænia hippocampi in the gyrus hippocampi; hence this convolution has a special connection with the optic thalamus through the fornix. In the corpus albicans the fibres of the fornix are arranged in loops, in the concavities of which nerve cells are situated. The optic thalamus also gives origin to the middle root of the optic tract. Owing to the connections of the locus niger, nucleus caudatus, and nucleus lenticularis with the crusta, Meynert has named them the ganglia of the crusta; whilst the optic thalami, corpora quadrigemina, and geniculata are the ganglia of the tegmentum. The comparison of the human brain with those of different

mammals has shown that the development of the hemispheres bears a direct relation to the size of the crusta and its ganglia, whilst the development of the hemispheres is in inverse relation to the size of the tegmentum and its ganglia.

The superior peduncles of the cerebellum connect that organ with the cerebrum. They arise in the grey matter of the vermiform process, ascend to the corpora quadrigemina, and some fibres are even prolonged apparently into the tegmentum, and through it doubtless into the optic thalamus.

b, The fibres which connect together the two hemispheres are called commissural fibres. The largest of these cominissures is the corpus callosum, which, as has already been described, connects corresponding convolutions in the opposite hemispheres. As its fibres lie on a plane superior to those of the corona radiata, the two systems of fibres intersect with each other on their way to the convolutions. The anterior commissure, though often described as connecting the two corpora striata, yet, as Spurzheim pointed out half a century ago, passes through these bodies to the convolutions around the Sylvian fissure, and gives a root of origin to the olfactory nerve. The posterior commissure passes into the two optic thalami; some of its fibres are said to extend into the tegmentum, and others into the substance of the hemisphere.

c, The tracts which connect different convolutions in the same hemisphere are named arcuate fibres, or fibræ propriæ. The arcuate fibres are situated immediately beneath the inner surface of the cortex of the hemispheres, and connect together the grey matter of adjacent convolutions. In some localities they are strongly marked, and have received special names.

The fasciculus uncinatus passes across the Sylvian fissure, traverses the claustrum and amygdala, and connects the convolutions of the frontal with those of the temporosphenoidal lobe. The fillet of the gyrus fornicatus extends longitudinally in that convolution, immediately above the corpus callosum, from its anterior to its posterior ends, and connects two different parts of its grey matter together. The longitudinal fibres of the corpus callosum, or nerves of Lancisi, also connect the anterior and posterior ends of the callosal convolution. The longitudinal inferior fasciculus connects the convolutions of the occipital with those of the temporal lobe. Longitudinal fibres lie on the inner surface of the septum lucidum, and extend into the gyrus fornicatus.

The corpora quadrigemina are connected with the optic thalami by nervous tracts called brachia, and smaller tracts also connect the thalami with the corpora geniculata. The peduncles of the pineal gland connect that body with the fornix, and are probably continued into the optic thalamus. The tania semicircularis is also at one end apparently connected with the optic thalamus, but its posterior termination is not well ascertained.

The great cerebral ganglia and the central masses of grey matter are centres of origin for sensori-motor nerves. The hemispherical ganglia, again, are the parts of the brain associated with the intellectual processes. The question has often been put, Are not the individual convolutions distinct organs, each endowed with special properties? and various arguments based on physiological, pathological, and anatomical grounds have been advanced in support of this proposition. In connection with the anatomical branch of the argument it may be stated that the convolutions possess, not only in man, but in all animals with convoluted brains, great regularity both in position and arrangement; but specialisation of form is not in itself a sufficient test of specialisation of function. Again, though the convolutions have definite forms they are not disconnected from each

other, for the grey matter forms a continuous layer over the whole surface of the hemisphere. Hence a group of cerebral convolutions differs from a group of muscles, each member of which is undoubtedly a distinct organ, for each muscle is isolated from those around it by a definite investing sheath. As regards internal structure, evidence has already been given that all the convolutions are not constructed on precisely the same plan, and it has also been pointed out that the convolutions are not all connected in the same way with the great cerebral ganglia. These structural modifications unquestionably point to functional differences in the several parts in which they are found. But further, special connections through the arcuate fibres are established between certain convolutions and not between others, and it is possible not only that particular combinations of convolutions through an interchange of internuncial fibres may condition a particular state of intellectual activity, but that these combinations associate various convolutions together in the performance of a given intellectual act, just as in the muscular system several muscles are as a rule associated together for the performance of a given movement. A clue to the special functions of the convolutions may perhaps be obtained by studying their connections, just as the action of the members of a group of muscles is ascertained by examining the direc tion of their fibres and the attachment of their terminal tendons.

MASS AND WEIGHT OF THE BRAIN.-The human brain Weight of is absolutely bigger and heavier than the brain of any brain. animal, except the elephant and the larger whales. It is also heavier relatively to the bulk and weight of the body than are the brains of lower animals, except in some small birds and mammals. Considerable variations, however, exist in the size and weight of the human brain, not only in the different races of mankind, but in individuals of the same race and in the two sexes. The heaviest brains occur in the white races. The average weight of the adult European male brain is 49 to 50 oz., that of the adult female 44 to 45 oz.; so that the brain of a man is on the average fully 10 per cent. heavier than that of a woman. The greater weight of the brain in man as compared with woman is not in relation merely to his greater bulk, but is a fundamental sexual distinction; for, whilst there is a difference of 10 per cent. in the brain weight, the average stature of women is, as Thurnam has calculated, only 8 per cent. less than that of men. Dr Boyd states that the average weight of the brain in the newly born male infant is 11.67 oz.; in the female only 10 oz. The exact age at which the brain reaches its maximum size has been variously placed at from the 3d to the 8th years by different authors; but it continues to increase in weight to 25 or 30, or even 40. After 60 the brain begins to diminish in weight; in aged males the average weight is about 45 oz., in females about 41 oz. In some cases the adult brain considerably exceeds the average weight. The brains of several men distinguished for their intellectual attainments have been weighed the brain of Cuvier weighed 64 oz.; of Dr Abercrombie, 63 oz.; of Professor Goodsir, 57 oz.; of Spurzheim, 55 oz. ; of Sir J. Y. Simpson, 54 oz. ; of Agassiz, 53.4 oz.; and of Dr Chalmers, 53 oz. But high brain weights have also been found where there was no evidence of great intellectual capacity. Peacock weighed four male brains which ranged from 62.75 to 61 oz.; Boyd, a specimen of 60.75 oz.; and Turner has recorded one of a boy aged fifteen which weighed 60 oz. In the brains of the insane high brain weights have also been observed. Bucknill met with a brain in a male epileptic which weighed 64 oz.; Thurnam, one which weighed 62 oz. ; and in the West Riding Asylum, out of 375 males examined, the weight of the brain in 30 cases was 55 oz. or upwards, and the highest weights were