The transverse fibres pass on either side into each hemisphere of the cerebellum, contributing with the processus cerebelli ad testes and the restiform bodies to form the crura cerebelli. They are the inferior peduncles of these crura. The anatomy of these transverse fibres evi dently denotes that they serve to connect the right and left cerebellar hemispheres, as cow mis.sures, and in a manner strikingly analogous to that in which the fibres of the corpus callosum connect the cerebral hemispheres. This view of the office of these fibres is strongly confirmed by the fact that their number is always in the direct ratio of the size of the lateral hemispheres, and that when the hemispheres are absent, these fibres no longer exist. When, therefore, the cerebellum consists only of a median lobe, there is no pons Varolii.
Some of the transverse fibres nearer the in ferior surface appear to dip in along the me dian line, and to pass upwards and backwards, forming a vertical plane of fibres which divides the mesocephale into two symmetrical portions, and Chaussier imagined that a decussation took place at this situation. The groove in. which the basilar artery lies is formed partly by the greater condensation which is produced along the median plane by this arrangement, and partly by the slight bulging on either side of it, caused by the ascent of the anterior py ramids. These fibres 'are continuous with a series of similar ones in the medulla oblongata (antero-posterior fibres of Cruveilhier).
The extent of the superior surface of the me socephale may be limited in front by a line which passes from side to side just before the anterior of the corpora quadrigemina, and pos teriorly by the base of the valve of Vieussens. This occupies a much greater space than the inferior surface. It is an inclined plane, and passes downwards and backwards, being con cealed by the anterior laminw of the superior vermiform process of the cerebellum and the posterior border of the corpus callosum.
The corpora or tubercula guadrigemina are four rounded eminences—gangliform bodies—' disposed in pairs (fig. 386, D, D'). The ante rior pair are larger than the posterior. The for mer have been distinguished. as the nates, the latter the testes.* These bodies are situate further forwards than the pons,and are chiefly connected with the superior surface of each crus cerebri.
The nates are of a deeper grey colour than the testes. In this respect they resemble the optic thalanai. Both pairs are similar in struc ture to those bodies. When cut into, they ap
pear to consist of fibrous matter intermingled with vesicular. Thin sections examined with the microscope exhibit intricate interlacements of tubular fibres with vesicular-matter inter posed—a true ganglionic structure.
An important fact deserves special notice as indicating that vesicular matter is found in these bodies in considerable quantity. The pia mater which adheres to their surface abounds in minute bloodvessels, and in sepa rating it these are seen to penetrate the tuber cles in vast numbers. This layer of pia mater contributes to form the velum interpositum.
The quadrigeminal,bodies are the analogues of the optic lobes in birds, reptiles, and fishes. In these classes there is only a single pair of tubercles. They are of considerable size in birds, and form a conspicuous portion of their encephalon. The division into four takes place only in Mammalia. The anterior are the larger in herbivorous animals, the posterior in the Carnivora. In most quadrupeds these bodies are concealed from view by the posterior lobes of the brain; but in Itodentia they are exposed in consequence of the imperfect developement of the brain in the backward direction.
The quadrigeminal bodies rest upon two processes of fibrous matter, which extend backwards to the median lobe of the cere bellum, and forwards to the optic thalami. These processes form a connection between the thalami and the quadrigeminal bodies and the cerebellum. They have been variously designated processus cerebelli ad testes, proces sus cercbelli ad corpora guadrigemina, processus cercbelli ad cerebrum.
The valve of Vieussens intervenes between these processes, and closes the fourth ventricle at its upper part.
A longitudinal groove separates the right and left pair of quadrigeminal bodies. The ante rior extremity of this groove forms an expanded and somewhat flattened surface on which rests the pineal gland (fig. 386, S). From the pos terior extremity a small band extends to the valve of Vieussens, called j'renuni. An inci sion made vertically downwards along the course of this groove exposes the canal through which the fourth ventricle and the third com municate (iter a terao ad portion ventricu lum). This canal communicates with the pos terior part of the third ventricle by an opening which is situate beneath the posterior commis sure, and with the superior extremity of the fourth ventricle beneath the valve of Vieussens.