OF THE MARSTJPIALIA AND MONOTREMATA. 
639 
The Rabbit. — Plate XXXVII. fig. 3 represents the inner surface of the cerebral hemi- 
sphere of a rabbit. The corpus callosum (B) is no longer horizontal in its general 
direction, but, like the upper margin of the hemisphere, is elevated at the posterior end. 
In front it is slightly thickened, but the rostrum is scarcely perceptible. Although 
this commissure in its median section appears elongated from before backwards, it is 
very thin from above downwards. The inferior layer of transverse (psalterial) fibres 
are well developed, and, except posteriorly, distinct from the main part of the great 
transverse commissure. The septal area is large in extent. The anterior commissure 
is proportionally larger than in man or in the sheep. The hippocampal sulcus, corre- 
sponding with the large size of its internal projection into the ventricle, is deep, and 
prolonged for some distance beneath the hinder end of the corpus callosum. The hollow 
for the reception of the optic thalamus and corpora quadrigemina is very large, and the 
fascia dentata (P) lying in it very broad. The smooth inner wall of the hemisphere 
shows no other sulcus than that of the hippocampus. 
The transverse section (Plate XXXVII. fig. 4) shows the corpus collosum at the 
bottom of the longitudinal fissure, curving up at the two extremities, in consequence 
of the form of the lateral ventricles. The anterior commissure is of actual greater 
depth in the section than the corpus callosum. Between the two is the septum, now 
only represented by the thick lower portion, very considerably increased in develop- 
ment. The thin upper part, together with the fifth ventricle, has entirely disappeared 
with the rostrum of the corpus callosum. 
In the Two-toed Sloth ( Cholcepus didactylus), Plate XXXVII. fig. 5, the same parts 
can be recognized, though somewhat changed in proportions. As compared with the 
sheep especially, the whole hemisphere is greatly shortened in the antero-posterior 
direction, and a greater shortening still has taken place in the corpus callosum. Instead 
of bearing, as in the sheep, the proportion to the hemisphere of 53 to 100, it is but as 
32 to 100. It rises at the posterior part, where it is slightly enlarged. The anterior 
end is simple and obtusely pointed, without a trace of the reflected rostrum. The 
anterior commissure is considerably larger, relatively to the hemisphere, than in the 
sheep. The ventricular aperture is nearly vertical in general direction. At the poste- 
rior edge of the body of the fornix there is a considerable thickening, caused by the 
transverse psalterial fibres of the corpus callosum. The hippocampal sulcus may be 
traced upwards to near the hinder end of the corpus callosum ; it then makes a sudden 
curve backwards, and almost immediately after another nearly equally sudden bend 
forwards, then arches over the end of the corpus callosum, and gradually approaching 
the upper surface of that body, at about its middle disappears in the lower margin of 
the callosal gyrus. Thus a thin portion of the dentate gyrus (fascia dentata) is continued 
over the hinder edge, on to the upper surface of the corpus callosum. In its principal 
part the gyrus itself is longitudinally grooved by a shallow sulcus, anterior and parallel 
to the hippocampal sulcus. The characteristic indentations are faintly indicated on the 
posterior edge. 
