THE CEREBELLUM. 707 



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 some- 

 what like the external layer of gray substance of the posterior lobes of the cerebrum 



Fio. 227. Cerebellum and medulla oblongata. (Hirschfeld.) 



1, 1, corpus dentatum ; 2, tuber annulare ; 3, section of the middle peduncle ; 4, 4, 4, 4, 4, 4, laminae forming the arbor- 

 vitae; 5, 5, olivary body of the medulla oblongata ; 6, anterior pyramid of the medulla oblongata; 7, upper ex- 

 tremity of the spinal cord. 



and is more or less sharply divided into two or more secondary layers. The most exter- 

 nal portion of this layer contains a few small nerve-cells and fine filaments of connective 

 tissue ; and the rest of the layer contains a great number of large cells, rounded or 

 ovoid, with two or three, and sometimes, though rarely, four prolongations. The mode 

 of connection between the nerve-cells and the fibres has already been described under 

 the head of the general structure of the nervous system. 



Course of the Fibres in the Cerebellum. Most anatomical writers give a very simple 

 description of the course of the nerve-fibres in the cerebellum. From the gray sub- 

 stance of the convolutions and their prolongations, the fibres converge to form finally the 

 three crura, or peduncles on each side. The superior peduncles pass forward and up- 

 ward to the crura cerebri and the optic thalami. These connect the cerebellum with the 

 cerebrum. Beneath the tubercular quadrigemina, some of these fibres decussate with the 

 corresponding fibres upon the opposite side; so that certain of the fibres of the superior 

 peduncles pass to the corresponding side of the cerebrum, and others pass to the cere- 

 bral hemisphere of the opposite side. 



The middle peduncles arise from the lateral hemispheres of the cerebellum, pass to 

 the pons Varolii, where they decussate, connecting together the two sides of the cere- 

 bellum. 



The inferior peduncles pass to the medulla oblongata and are continuous with the 

 restiform bodies, which, in turn, are continuations chiefly of the posterior columns of the 

 spinal cord. 



From the above sketch, the physiological significance of the direction of the fibres, 

 as it appears from the most reliable and generally-accepted anatomical investigations, is 

 sufficiently evident. By the superior peduncles, the cerebellum is connected, as are all 



