Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/936

Rh ANATOMY [NERVOUS SYSTEM clusters of minute granules and a compact arrangement of small nerve cells. 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 eras cerebri. The corpus ~striatum cerebri consists of two masses of grey matter separated from each other by numerous stride 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 striae. The nerve cells in the grey matter are both multipolar and fusiform. The external corpus geni- culatum 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 cms 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 cerebro- spinal 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 com missure, 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 com missure ; 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 cere brum with the lower divisions of the encephalon; 6, 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 thalami fibres radiate into the convolutions of the lobes of 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 inter position 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 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. Tho crusta forms the superficial or anterior part of the crus. Its fibres are in greater part continuous with the longi tudinal 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 upv/ards 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 ihe 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 tsenia hippo campi in the gyrus hippocampi ; hence this convolution has a special connection with the optic thalamus through the forniz. In the corpus albicans the fibres of the fornix are arranged in loops, in the concavities of which nerve cella 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 lenticu laris with the crusta, Meynert has named them the ganglia of tiie crusta ; whilst the optic thalami, corpora quadrige mina, and geniculata arc the ganglia of the tegmentum. The comparison of the human brain with those of differed