272 INNERVATION. [CHAP. X. 



on a different plane. These are called its peduncles. Through 

 them the cerebellum forms a connexion with other parts of the 

 encephalon. The superior layer or peduncle is a bundle of fibres 

 which extends to the corpora quadrigemina, processus cerebelli ad 

 testes ; the middle layer passes to the medulla oblongata, the resti- 

 form bodies ; and the inferior peduncle consists of transverse fibres, 

 (Pom Varolii), which pass to the opposite side, and also form a 

 considerable portion of the mesocephale. 



Lesions of the cerebellum, when so deep-seated as to affect the 

 primary planes of fibrous matter or the central stem, have the same 

 crossed effect as those of the cerebrum. This is not so obviously 

 explicable as the similar instance of the cerebrum, for the cerebellar 

 fibres of the medulla oblongata (restiform bodies) do not appear to 

 decussate. Yet it seems scarcely necessary, in order to explain 

 the phenomenon, to have recourse to the supposition that they do 

 decussate. The close connexion between the restiform bodies and 

 the pyramids, by means of the arciform fibres, renders the latter ex- 

 ceedingly liable to sympathise with the condition of the former, and, 

 therefore, prone to propagate the morbid influence to the opposite 

 half of the spinal cord, and through that to the opposite half of the 

 body. It must be borne in mind that some of the fibres of the an- 

 terior pyramids very probably derive their origin from the central 

 gray matter of the medulla oblongata, as well as of the mesoce- 

 phale, and that some, at least, of those which affect the right half 

 of the cord, probably derive their origin from the left side of either 

 or both of those segments of the encephalon. That lesion of one 

 hemisphere of the cerebellum may influence the corresponding half 

 of the medulla oblongata, is likely, from the connexion which the 

 restiform fibres establish between them. The connexion of the 

 cerebellum with the mesocephale is most clearly established by 

 means of the transverse fibres which constitute the Pom Varolii. 



Respecting the intimate structure of the cerebellum, little is known 

 of a very exact nature. The white stems and plates are fibrous, 

 and consist of multitudes of nerve- tubes of all sizes, which follow 

 the general direction of each stem or plate. These fibres doubtless 

 tend principally to propagate the peculiar influence of the cerebel- 

 lum to the spinal cord and the mesocephale. Some probably are 

 commissural, as the processus cerebelli ad testes (or cerebro-cere- 

 bellar commissures), the fibres of the pons, and some of those of the 

 median lobe. Mr. Mayo supposes that others pass between the 

 laminae, but their existence is extremely doubtful. 



The vesicular matter which covers the plates contains the ordi- 



i 



