322 STRUCTURE OF THE CEREBELLUM. 



The cerebel- arises, as has been stated, from the triple strand of the eras 

 lum. cerebelli, of which one layer of fibres is connected with the 



corpora quadrigemina, and through them with the optic thalami ; a sec- 

 ond with the restiform bodies ; and the third is commissural, and passes 

 forward as the pons varolii. 



Like the cerebrum, this organ is vesicular on its surface, which pre- 

 sents a number of parallel lines, which are fissures descending to the in- 

 terior. Their object is apparently the same as that of the convolutions 

 of the brain, the augmentation of surface. Of these fissures, the deep 

 are termed the primary : they divide the organ into lobes. Those which 

 descend to a less depth are termed secondary : the divisions they give 

 rise to are lobules. The gray vesicular material does not, however, de- 

 scend to the bottom of the primary fissures, and in this respect they dif- 

 fer from the cerebral convolutions. Moreover, from this circumstance, 

 that material is not continuous all over the cerebellum, but is in divided 

 portions. 



Such are the appearances presented on an exterior examination of 

 Structure of ^ ie cerebellum. Viewed as a development upon the crura 

 the cerebellum, cerebelli, it may be described as consisting of a median lobe 

 and two hemispheres ; the former is, however, found existing alone in fish- 

 es and reptiles, the latter being subsequently added in the higher tribes. 

 From the central column of each hemisphere white fibrous planes are 

 given off, and from these, again, secondary, and again, tertiary planes 

 proceed. The planes are covered with vesicular matter, and thus give 

 rise to the appearance spoken of in the preceding paragraph, in the exte- 

 rior examination of the cerebellum, as primary and secondary fissures.' 

 They are lined with pia mater. The median lobe is formed on the same 

 plan. Its fibrous stem comes from the processus cerebelli ad testes, or, 

 more properly, from the optic thalamus. The weight of the cerebellum, 

 compared with that of the cerebrum, is usually stated as being about 1 to 8. 



Much diversity of opinion prevails respecting the true function of the 

 cerebellum, some supposing that it is the centre of common sensation, 

 others that it is for the purpose of co-ordinating muscular movement, 

 and others that it is the seat of sexual instinct. 



That the cerebellum is one of the sensory ganglia may be inferred from 

 Function of the the history of its development and its anatomical connec- 

 cerebeiium. tions. Its median lobe is the first to appear, as in fishes, 

 and the hemispheres arise subsequently as appendages thereto, as in 

 birds. The size which these eventually attain gives them a deceptive 

 prominence, and hides their subordinate character. Regarding the lobe, 

 therefore, as the essential and fundamental portion of the structure, the 

 significance of its cerebral connection with the thalamus through the pro- 

 cessus ad testes is too obvious to be overlooked. As by this its senso- 



