THE CEREBELLUM. 779 



1. External Conformation of the Cerebellum (Figs. 423, 433). 



The cerebellum — isolated by dividing its lateral peduncles from the medulla 

 oblongata on which it is fixed— is almost globular in form, slightly elliptical, and 

 elongated trans\-ersely ; while its external surface is furrowed by a great number 

 of sulci, the two principal of which {sulci horizonlalis) 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 consecjuence of the shallowness and irregularity of the two sulci 

 separating them. We will, nevertheless, study them in succession, and afterwards 

 examine— in a general manner — the furrows on their superficies. 



Middle lobe (Fig. 423, c^). — This has been compared to a silkworm rolled 

 in a circular manner around the middle portion of the cerebellum, with its two 

 extremities joined — without being confounded — below the inferior face of the 

 organ. 



This vermicular disposition is not well defined in the middle and superior 

 portions of the cerebellum, where this lobe is always more or less subdivided into 

 large multiple and irregular lobules ; but it is better marked before and behind, 

 in those points which correspond to the two extremities of the creature selected 

 as a term of comparison. There may be remarked two longitudinal eminences 

 transversely annulated on their surface, 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, the roof of which they concur in forming. 



On the anterior vermiform p-ocess the posterior border of the valve of 

 Yieussens is inserted. 



The posterior rermicidar ])rocess also receives the insertion of a valve already 

 mentioned, and which must be again briefly referred to. This valve — described 

 for the first time by Renault — forms a lamina of a certain thickness stretched 

 above the calamus scriptorius. It has exactly the triangular form of this space, 

 and presents a superior face covered by the posterior vermiform process ; an 

 inferior face, studded in some points with small vascular loops ; a base fixed to 

 the vermis, near the free extremity of that prominence, and to its lateral parts ; 

 two lateral borders, attached to the corpora restiformia on each side of the 

 calamus scriptorius ; and a summit corresponding to the receding angle of the 

 excavation. This lamina is, doubtless, nothing more than a septum formed by 

 the external pia mater, and on which is extended the internal membrane that 

 hues the walls of the cerebellar ventricle. Otherwise, it is in direct continuity, 

 towards its base, with an evident dependency of the pia mater — the plexus 

 char aides. ^ 



Lateral lobes (Fig. 423, C^, C^). — These are shaped like two irregular seg- 

 ments of a sphere. Their surface, fissured and lobulated in every direction, 

 presents nothing interesting externally, superiorly, or posteriorly. It is by 

 their inferior part that the peduncles enter the substance of the cerebellum ; 

 and behind this point, beneath their lateral parts, lies the cerebellar plexus 

 choroides. 



The cerebellar choroid plexuses. — This name is given to two small reddish 

 granular masses, formed of vascular loops, elongated from before to behind, 



' This Beptum is represented in the rudimentary state in Man, by the vcUvtUx Tarini. 



