NERVOUS SYSTEM. 323 



Lobes. — The cerebellum is superficially divided into three ob- 

 long /obes — a middle lobe, and two lateral lobes. The lateral 

 lobes, which are opposed to the posterior lobes of the cerebrum, 

 are small and ovoid. The middle lobe is prominent, arched, and 

 subdivided into lobuli by several sulci, conspicuous for their 

 depth, crossing its upper and under surfaces. The anterior and 

 more prominent part of this lobe, which is received into the great 

 fissure of the cerebrum, is denominated the aiiterioi- vermiform 

 process J the continuation of it, along the summit of the head, 

 forming the posterior vermiform process. 



Arrangement of component Substances. — In the cerebellum, the 

 disposition and pro|)ortioiiate quantities of medullary and cortical 

 matters are reversed, with regard to the cerebrum. Herein the 

 cortical substance exceeds the medullary in quantity, and, in- 

 stead of forming the outward parts, pervades the innermost. 

 Slice the cerebellum horizontally, and these substances present a 

 laminated appearance : on the other hand, make a vertical sec- 

 tion of it, and they are found so disposed as to give the surface 

 an arborescent representation, usually denominated the arbor 

 vita. 



Fourth Ventricle. — Within the cerebellum, or rather between 

 it and the tuber annulare and medulla oblongata (which two last 

 form its back part), is situated the fourth ventricle. Beginning 

 above the testes, where it is perforated by the iter a tertio, it ex- 

 tends upward and backward to the extremity of the medulla ob- 

 longata, where it ends in a sharp angular point, which, with a 

 groove issuing from it and continued along the floor of the ven- 

 tricle, anatomists have resembled to a pen, and thence called the 

 calamus scriptorius. 



Valvula Vieussenii. — Extended across the inferior and anterior 

 part of the ventricle, closing the cavity between the testes and 

 lower extremity of the anterior vermiform process, is a membrano- 

 medullary curtain, the velum vel valvula Vieussenii. It is 

 supported at the sides by two medullary productions, remarkable 

 for their whiteness, the processus a cerebello ad testes, which some 

 regard as the colunuis of the valve. 



The choroid plexus of the cerebellum is found, lying crosswise, 

 within the posterior part of the fourth ventricle, between the cere- 

 bellum and medulla oblongata. Jt nowise differs in its composi- 

 tion from the plexuses of the lateral ventricles. It is distributed 

 into three principal divisions: one lies in the middle of the cala- 

 mus ; the other two (lateral) are lodged within fissures in the 

 cerebellum, occupying the spaces between it and the tuber an- 

 nulare. 



