The white matter consists exclusively of fibres, chiefly of the tubular kind and of all degrees of size. These, in the more distant ramifications, penetrate the vesicular matter of their grey cortex, and form some unknown con nection with its elements. The grey matter consists of three layers, readily distinguishable by the naked eye from their difference of co lour. The external layer is the darkest, and consists chiefly of granular and vesicular mat ter. The next or intermediate layer is of a light colour, and is composed of a stratuin of fine nucleus-like particles. The third layer has the greatest thickness, and is immediately in con tact with the fibrous matter; it is intermediate in point of colour to the other two, and con sists of numerous vesicles of the caudate kind, especially with branching processes and nerve tubes of all sizes. The dark colour of the external layer is doubtless owing in a great measure to the great numbers of capillary ves sels which enter it; the greater paleness of the inner stratum is to be attributed to the inter mixture of the white fibres, whilst the light colour of the middle stratum is intrinsic. From the usual dependent position of the cerebellum in the dead body, it always appears to contain more blood than the cerebrum.
Corpus dentatum.—lf, in making a vertical section of either hemisphere of the cerebellum, the incision be made so as to leave two-thirds of the hemisphere on its outside, a peculiarity will be observed on the surface of the section which deserves a separate consideration. The central white nucleus is interrupted by a very remarkable undulating line of vesicular matter, which is convex towards the posterior margin of the hemisphere, but open in front towards the crus cerebelli.
This constitutes the corpus de-atation or rhomboideum of the cerebellum. It presents a remarkable resemblance to the structure of the same name which is met with in the olivary body of the medulla oblongata. It is evidently a capsule of vesicular matter which is enclosed in the inner third of the substance of the centml white nucleus of the cerebellar hemisphere, being nearer its superior than its inferior sur face. The peculiar undulating arrangement of it doubtless has reference to the accommoda tion of a certain extent of surface in a limited space. The fibrous matter enclosed by it seems derived frorn the processus cerebelli and flour the restiform body.
The centml stem of fibrous matter to which the several lobules, both of the hemispheres and the median lobe of the cerebellum, adhere, ( crus cerebelli,) is formed by three bundles of fibres, each situate on a different plane. These are the peduncles of the crus cerebelli. Through them the cerebellum forms a connection with other parts of the encephalon. The superior layer or peduncle is a bundle of fibres which extends to the corpora quadrigemina, and may be traced beneath them to the optic thalami. These are the processus cerebelli ad testes, but from their being obviously a medium of con nection between the cerebellum and the cere brum, they may be better named cerebro-cere bettor commissures. It is worthy of remark,
that these are the only fibres which appear to connect these two segments of the brain. The rniddle layer is continuous with the restiform bodies, processus cerebelli ad medullam oblcm gatam. And the inferior layer is evidently de rived from the tmnsverse fibres of the pans Varolii, which thus pass from one hemisphere to the other, and constitute a great commissure to the cerebellar hemispheres. These fibres, moreover, connect each hemisphere to the me socephale (fig. 380, t, r, v).
From this triple constitution of the crus cerebelli, it is plain that the cerebellum may exert an influence upon, or be affected by the optic thalami or quadrigeminal bodies, the restiform columns, or the mesocephale.
Of the fourth ventricle.—This is a rhomboi dal cavity, situated at the upper and posterior part of the medulla oblongata, and extending over part of the superior surface of the meso cephale. It is limited superiorly by the poste rior margin of the testes, and inferiorly by the superior blunt extremity of the posterior pyra mids. Its two lateral angles correspond to the entrance of the restiform bodies into the crura cerebelli. In fact, it is formed by the diver gence of the restiform colutnns in their ascent to the hemispheres of the cerebellum. The median lobe of the cerebellum lies over the fourth ventricle, and conceals it from view. The anterior lohnle of the inferior vermiform process, the nodule, projects into it, and closes it below. On either side of this lobule a process of pia mater, with small granulations upon it, is found. These processes are the choroid plexuses of the fourth ventricle. A round these and thence on to the nodule, the proper membrane of the ventricle is reflected, and thus its cavity is shut out from any com munication with the subarachnoid cavity. A vertical section in the median plane, or a little to one side of it, displays this arrangement well. ( Fig. 392.) Along the floor of the fourth ventricle we find the central or olivary columns of the me dulla. oblongata extending upwards to the optic thalami. A fissure, continuous with the pos terior median fissure, separates these columns, and terminates above in a canal which pene trates the mesocephale, to reach the third ven tricle iter a tertio ad quartum ventriculuni or aqueduct of Sylvius. On either side of the fissure certain bundles of white fibres, conti nuous with the auditory nerves, join it at right angles, crossing over the olivary columns. This fissure, with its white fibres on each side, has been compared to a pen with its barbs, and hence called calamus scriptarius.
The fourth ventricle, although sometimes called the ventricle of the cerebellum, properly belongs to the medulla oblongata. It is present in all the vertebrate classes, and in size bears a direct proportion to that of the medulla itself.