360 NEKVOUS SYSTEM. 



tionate weight to that of the cerebrum is as 1 to 8-f- in the 

 male, and as 1 to 8J in the female. It is separated from the 

 cerebrum by a strong process of the dura mater, called the 

 tentorium. Like the cerebrum, the cerebellum presents an 

 external layer of gray matter, the interior being formed of 

 white, or fibrous nerve-tissue. The amount of the gray sub- 

 stance is very much increased by numerous fine convolu- 

 tions, and is farther extended by the penetration, from the 

 surface, of arborescent processes of gray matter. Near the 

 centre of each lateral lobe, embedded in the white substance, 

 is an irregularly dentated mass of cellular matter, called the 

 corpus dentatum. The cerebellar convolutions are more 

 numerous, and the gray substance is deeper, than in the 

 cerebrum ; and these convolutions are present in many of 

 the inferior animals in which the surface of the cerebrum is 

 smooth. 



The cerebellum consists of two lateral hemispheres, more 

 largely developed in man than in the inferior animals, and a 

 median lobe. The hemispheres are subdivided into smaller 

 lobes, which it is unnecessary to describe. Beneath the 

 cerebellum, bounded in front and below by the medulla 

 oblongata and pons, laterally by the superior peduncles, and 

 superiorly by the cerebellum itself, is a lozenge-shaped 

 cavity, called the fourth ventricle. The crura, or peduncles 

 will be described in connection with the direction of the 

 fibres. 



The structure of the gray substance of the convolutions 

 presents certain peculiarities. This portion is divided quite 

 distinctly into an internal and an external layer. The inter- 

 nal layer presents an exceedingly delicate net-work of fine 

 nerve-fibres, which pass to the cells of the external layer. 

 In the plexus of anastomosing fibres, are found numer- 

 ous bodies like free nuclei, called by Robin, myelocytes. 

 The external layer is somewhat like the external layer of gray 

 substance on the posterior lobes of the cerebrum } and is 

 more or less sharply divided into two or more secondary 



