DISSECTION OF THE BRAIN, OR ENCBPHALON. 241 



portions have a distinct resemblance to two caterpillars, and they may 

 be distinguished as the anterior and jyosterior vermiform jyrocesses. The 

 anterior vei'miform process is adherent to the valve of Vieussens. Each 

 lateral lobe is joined on its inferior aspect by three bundles of nerve 

 fibres, which are termed the ^:)efZimc^es. The 2)osterior peduncle is the 

 termination of the restiform body, the middle peduncle is the reflected 

 extremity of the pons, and the anterior pednncle passes forwards beneath 

 the corpora quadrigemina. 



Besides the grooves which divide the cerebellum into its lobes, 

 numerous smaller fissures occur over its surface, and divide the lobes 

 into folia, or leaflets. The arrangement of these leaflets will be made much 

 more evident by making an antero-posterior vertical section, at or near 

 the mesial plane of the organ. The peduncles are to be cut as they 

 enter the lower face of the lateral lobe, and the anterior vermiform 

 process is to be carefully separated from the valve of Vieussens Avith the 

 scalpel. This will enable one half or a little more of the cerebellum to 

 be removed after the manner of Plate 35. 



The cerebellum contains both grey and white matter. The white 

 matter forms a large mass in the interior, and from this mass large plates 

 are given off" towards the surface. From these primary plates proceed 

 more numerous smaller secondary plates, and these again detach small 

 terminal plates which end in the surface folia. In consequence of this 

 disposition of the white matter, it presents on vertical section a strikingly 

 arborescent appearance, to which the term arbor vitm is applied. The 

 nerve fibres of the white matter are for the most part directly continuous 

 with the peduncles; but some are proper to the organ, and comiect 

 different points of the grey matter. 



The grey matter of the cerebellum is spread over its surface, and also 

 forms two independent masses within the central mass of white matter. 

 These latter have the form of a corrugated capsule, and each is placed a 

 little to one side of the mesial plane, and is known as the corptis dentatum 

 of the cerebellum. The surface layer of gi'ey matter invests the core 

 of white matter within each leaflet, and also extends across the bottom 

 of the fissures between adjacent leaflets. It consists of two strata : an 

 outer grey layer, and an inner rust-coloured layer. 



The Fourth Ventricle (Plates 35 and 36). This is a space between 

 the cerebellum above, and the medulla and pons below. Its boundaries 

 are as follows : — Its floor is formed by the medulla and pons ; its roof 

 by the valve of Vieussens, the under suface of the vermiform lobe, and 

 the reflection of pia mater from the medulla to the cerebellum ; laterally 

 it is bounded in its anterior third by the anterior peduncle of the cere- 

 bellum, and in its posterior two-thirds by the restiform body. The 

 widest part of the space is at the point where the peduncles enter the 

 cerebellum, and it contracts towards both extremities. At the posterior 



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