The hippocampi, major and minor, are constant convolutions, which project into the lateral ventricles, the latter into its posterior, the former into its descending horn.
In general the constituent fibres of the white matter of the convolutions converge from the inner surface of the cortical layer to the cen trum ovate, or if followed from the centrum ovate, they radiate to the grey surface, whether of a convolution or of a sulcus. A remarkable exception is in the case of the internal convo lution,- the fibrous matter of svhich constitutes, as has been already explained, a longitudinal commissure. The thickness of the cortical layer is pretty uniform, at least relatively to the size of the folds themselves. Throughout its entire thickness it is mixed with fibres, which are most numerous at its adherent surface, but extremely few and scattered at its free surface.
In hydrocephalus the convolutions disap pear. The fibrous matter becomes greatly ex panded by the fluid accumulated in the ven tricles, and when its expansion has gone so far as to equal the grey surface, the folded cha racter of the latter disappears. This takes place precisely in the same way that the rugm of the contracted stomach (as before referred to) become obliterated when the muscular coat re laxes and allows the full distension of the °ma, n.
Mayo supposes that other fibres are found in the convolutions besides those which are conti nued into the centrum orate. These are com missural ones, which pass from convolution to convolution--either between adjacent or dis tant ones—forming arches the convexities of which are directed to the centrum ovate. I have never succeeded in satisfying myself of the existence of such fibres either in the fresh brain or in that preserved in spirit. If they exist, it is evident that they must be commis sural between particular convolutions. The same anatomist supposes that similar commis sural fibres connect the larninm of the cere bellum.
The principal bulk of the hemispheres is formed by fibrous substance. This is shown by the horizontal section which displays the centrum ovate. These fibres radiate from those surfaces of the optic thalami and corpom striata which are in contact with the substance of the hemisphere. Most of the fibres which emerge from these gangliform bodies pass to the gre matter of the convolutions. Some, howeve turn inwards towards the mesial plane, an form the corpus callosum by their union wi those of the opposite side.
It cannot be supposed that all the remainio fibres, after subtracting those which form th corpus callosum, pass through the,thalami an corpora striata. The disproportion of numbe
betcveen the fibres of the medulla oblonga and these is too striking to allow such an hy pothesis. They mingle with the vesicular ma ter of both; some do not pass beyond them others are continued into the medulla oblo gata, either to its olivary or its pyramidal c lumns.
Corpora striata and optic thalami.—TI corpora striata and optic thalami bear a stro re,semblance in general character and structu to ganglia. They are ovoid masses placed h tweet) the fibrous substance of the hemisphe on the one hand, and the medulla oblongata o the other. These bodies, which are best dis played by laying open the lateral ventricl (p. 675), are very closely united to each othe The corpus striatum is placed a little in gr.d to the outside of the thalamus. It is pear shaped : its thick end is directed forwards and inwards, and it gradually tapers backwards into a caudate process of considerable length, which winds downwards, forwards, and inwards into the descending cornu of the lateral ventricle, at the anterior extremity of which it terminates. Placed on the outside of the thalamus, it seems to embrace it there, and to adhere very inti mately to it. The tmnia semicircularis lies in a groove between the two bodies, and as it vvere constricts their connecting fibres.
The corpus striatum is of a dark grey colour. A considerable portion of it projects free into the cavity of the ventricle, forming an extensive convex surface there. The rest is firmly im bedded in the fibrous substance of the hemi sphere, and in position corresponds to the base of the insula, which for that reason has been called the tubule of the corpus striatum. The free surface as contributing- to form the ventri cular wall is covered by the lining membrane of the ventricle and a layer of nucleus-like particles; it is traversed by several veins. This surface is limited on the outside by the plane of fibres, which, after emerging from it, incline inwards and contribute to form the corpus cal losum. On the inside it is limited by the taania semicircularis, which separates it from the optic thalamus. That portion of the free surface which is seen in the inferior horn of the ven tricle has, as already stated, the appearance of a caudiform prolongation of the upper portion; this probably arises from the diminution of the body in thickness at its inferior part, the portion which belongs to the inferior cornu forming the apex of a cone, of which the upper convex portion forms the curvilinear base.