The most complete refutation of this assertion is afforded by M.Leuret's series of observations of the brains of geldings and entire horses. These researches, indeed, shew that in stallions the cerebellum is proportionally smaller than in mares or geldings, and that in geldings it is larger than in mares. It is very evident from them that mutilation of the sexual org,ans does not cause degeneration of the cerebellum.
The shape of the cerebellum is that of " an ellipsoid flattened from above downwards." Its principal diameter, which is transverse, is from three-and-a-half to four inches in length ; the antero-posterior diameter is from two in ches to two inches and a half; the anterior part is about two inches in thickness; whilst near its posterior edge it does not measure above half an inch.
At its anterior edge the cerebellum is notched, and receives fibres by which it is connected to the cerebrum and mesocephale. This notch is of considerable transverse extent, and is semi lunar in shape. The greater portion of the posterior part of the mesocephale corresponds to it. By Reil this is called the semilunar _fis sure. In it we find several parts which the anatomist should study; namely, on the highest plane, the processus cerebelli ad testes, sepa rated by the valve of Vieussens, and, beneath these, the fibres of the restiform columns, and the right and left extremities of the pons Varolii, all of which combine to form the crus cerebelli or centml stem of each lateml lobe.
The posterior margin is interrupted in its middle by a vertical notch, which divides it into a right and left portion. This notch is wider ill front than behind, whence Reit called it the purse-like ,fissure. The term posterior notch is preferable. It receives the falx cere belli, and at its bottom we observe a continuity between the superior and inferior laminw of the median lobe of the cerebellum.
The superior surface of the cerebellum is slightly convex, inclined backwards and down wards. It terminates in front by a concave margin, which overlaps the parts contained in the semilunar fissure. This surface is more convex along the middle than on either side. In the latter situations it is inclined and nearly plane ; but in the former it resembles more the surface of a cylinder. This middle portion
corresponds to what is commonly called the superior vermiform process : it is in fact the upper surface of the median lobe of the cere bellum.
On its inferior surface the subdivision of the cerebellum into two symmetrical portions is very apparent, by reason of the existence of a deep fissure which proceeds from before back wards along the median line, and is continuous behind with the posterior notch. This fissure is called the valley (vallecula, Haller; grande scissure mediane du cervelet). It separates the hemispheres of the cerebellum, each of which presents a very convex surface, corresponding to each occipital fossa. The arachnoid mem brane is extended from one to the other, towards the posterior part of the fissure, leaving a con siderable space between it and the pia mater, which is traversed by some fine bundles of fibrous tissue and occupied by subarachnoid fluid. This space has already been referred to as the posterior conflux of Majendie.
The anterior part of this fissure receives the upper and posterior portion of the medulla oblongata. The remainder of it is occupied by the inferior surface of the median lobe of the cerebellum, presenting a remarkable cru ciform arrangement, which will be presently described.
Another very remarkable fissure requires a special notice. It is horizontal, and passes into the substance of the cerebellum, dividing it into an upper and an inferior portion. By inserting the handle of a knife along the pos terior margin of the cerebellum, this fissure may be shewn to pass forwards to a consider able depth, and to communicate on each side with the semilunar fissure, vvhilst it is inter rupted in the iniddle posteriorly, by the notch. Its inner surface is lined by a process of pia mater, which sinks into it.
The right and left cerebellar hemispheres exhibit a general synnnetry, which is, however, not always perfect, as a manifest difference is sometimes observable in their sizes. And a corresponding want of symmetry may be fre quently seen in the right and left fossm of the occipital bone.