The fourth pair of nerves are seen upon this surface, attaching themselves to the pmcessus cerebelli ad testes, or to the Vieussenian valve, or to the posterior pair of quadrigeminal bodies.
Besides the anterior pyramids, the olivary columns are continued through the mesoce phale to form with the former the crura cerebri. These columns are exposed along the floor of the fourth ventricle; higher u p, however, they are surrounded by a lightish grey matter, form the superior stratum of each crus cerebri, separated from the quadrigeminal tubercles by the pro cessus cerebelli, and finally merge into the optic thalami. Their course is well displayed in jig. 380, where j represents the olivary columns, t the processus ccrebelli ad testes, and v the pons penetrated by p, the pyramids.
The olivary columns retain their greyish hue in their upward course. Their cylindrical form is very apparent on the floor of the fourth ven tricle; but it is still more obvious on viewing a transverse section, when each olivary column appears as a cylinder, to be distinguished from the rest by its roundness and its peculiar colour.
No other mode of dissection conveys so much knowledge of the anatomy of this part as a transverse section, carried from above downwards through either pair of quadrige minal bodies, and inclined a little backwards, so as to pass through the polls. The parts which may be observed on such a section, enumerated from above downwards,—are, 1, either pair of quadrigeminal tubercles ; 2, between and beneath them, the iter cut across; 3, on either side of this, fibrous matter ; 4, below this on each side, the section of each olivary column ; 5, planes of transverse fibres interlacing with longitudinal ones, and grey matter between the planes; 6, transverse fibres forming the pons Varolii.
From the preceding description of the me socephale it may be concluded that two classes (' of elements enter into its formation. These
are intrinsic and extrinsic. The former con sists in the masses of vesicular matter, with which the fibrous matter, whatever be its course, is intimately connected. Such are the grey matter of the quadrigeminal bodies; that light grey matter which surrounds the olivary co lumns in their upward course ; the darker matter which intervenes between the transverse fibrous lamell; and more in front, that which forms the locus niger of the crus cerebri.
The extrinsic elements are those which pass through this segment, being continuous with some portion of a neighbouring segment, or serving to connect the grey matter of the me socephale with the hemispheres of the cere brum or cerebellum, or with the medulla oh longata. The fibres which forrn the inferior layer of the pons are perhaps the only element that does not connect itself in some way with the grey matter of the mesocephale, since they seem simply to pass across from one crus cerc belli to the other. The deeper transverse fibres, the pyramids, the olivary columns, the pio cessus cerebelli ad testes, all connect neigh bouring parts with the intrinsic matter of the inesocephale.
It is plain, then, that anatomy affords abun dant grounds for the conclusion, that the me socephale must be regarded as a distinct centre, connected by numerous bonds of umon with the other segments of the brain.
If further proof of this were wanting, it would be found in the connexion of two im portant nerves with this segment. These are the fifth and the fourth pair. The former pene trate between the superficial fibres of the pons which spread out upon the crus cerebelli; the latter are connected with the superior surface of the mesocepliale.