686 THE CENTRAL AXIS OF THE NEEVOVS SYSTEM. 



prominent than in animals, and are also distinguished by the presence of a grey 

 nucleus in their interior. The medulla oblongata of Man has not the transverse band, 

 behiud the pons Varolii, which we described in the Horse (Fig. 333). 



With regard to the isthmus proper, it contains the parts in front of the medulla 

 oblongata already studied in the domesticated animals. The pons Varolii is very large ; 

 the crura cerebri are separated from each other by a groove, at the bottom of which are 

 several small openings. The fourth ventricle is deep, is bordered by well-developed 

 corpora restiformia, and inclosed posteriorly and laterally by the valves of Tarin {velum 

 medullare posterius). On its floor are remarked transverse striaa Qinex transversa;) named 

 the barbs of the calamus seriptorius, which are also found in the Dog. The testes are 

 smaller than the nates'; but the diiference in their volume is less considerable than exists 

 between Solipeds and Euminants. Their structm-e is already known. 



Aetiolb III. — The Cbeebellum. 



The cerebellum, or posterior enlargement of the encephalon, is the 

 single mass supported by the isthmus, separated from the cerebrum by the 

 transverse partition constituting the tentorium cerebelli, and lodged in the 

 posterior compartment of the cranial cavity, vphich almost exactly gives the 

 measure of its volume. 



1. External Conformation of the Cerebellum. 



The cerebellum, isolated by dividing its lateral peduncles from the 

 medullary prolongation on which it is fixed, presents the form of an almost 

 globular mass, slightly elliptical, elongated transversely ; while its externall 

 surface is furrowed by a great number of sulci, the two principal of which 

 (sulci Jiorizontalis) pass in a circular manner on each side of the middle line 

 around the organ, dividing it into three lobes— a, middle and two lateral. 



The three lobes of the cerebellum are not always readily distinguished 

 from each other, in consequence of the shallowness and irregularity of the 

 two sulci separating them. "We will, nevertheless, study them in 

 successipn, and afterwards examine, in a general manner, the furrows on their 

 superficies. 



Middle lobe (Fig. 321, 2).— This has been compared to a silk-worm 

 rolled in a circular manner around the middle portion of the cerebellum, 

 and whose two extremities are joined, without being confounded, below the 

 inferior face of the organ. 



This vermicular disposition is not well defined in the middle and 

 superior portion of the cerebellum, where this lobe is always more or less 

 subdivided into large multiple and irregular lobules ; but it is better 

 observed before and behind, in those points which correspond to the two 

 extremities of the animal selected as a term of comparison. There may 

 be remarked two longitudinal eminences transversely annulated on their 

 sm-face, and curved beneath the cerebellum in such a way as to come in 

 contact with each other. These eminences constitute the anterior and 

 posterior vermiform processes. Their extremities are lodged in the fourth 

 ventricle, whose roof they concur in forming. 



On the anterior vermifwrn process the posterior border of the valve of 

 Vieussens is inserted. 



The posterior vermicular process also receives the insertion of a valve 

 already mentioned, and which must be again briefiy referred to. This valve, 

 described for the first time by M. Eenault,' forms a lamina of a certain 

 thickness stretched above the calamus seriptorius. It has exactly the 

 triangular form of this space, and presents a superior face covered by the 

 Q The late eminent veterinary teacher and director of the Alfort School.) 



