624 NERVOUS SYSTEM. 



fourth ventricle. The crura, or peduncles, will be described in connection 

 with the direction of the fibres. 



The gray substance of the convolutions is divided quite distinctly into an 

 internal and an external layer. The internal layer presents an exceedingly 

 delicate net-work of fine nerve-fibres which pass to the cells of the external 

 layer. The external layer is somewhat like the external layer of gray sub- 

 stance of the posterior lobes of the cerebrum and is more or less sharply di- 

 vided into two or more secondary layers. The most external portion of this 

 layer contains a few small nerve-cells and fine filaments of connective tissue. 

 The rest of the layer contains a great number of large cells, rounded or ovoid, 

 with two or three and sometimes four prolongations. The mode of connec- 

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

 the head of the general structure of the nervous system. 



Directions of the Fibres in the Cerebellum. Fibres from the gray sub- 

 stance of the convolutions and their prolongations, and from the corpus denta- 

 tum, converge to form the three cerebellar peduncles on either side. The 

 superior peduncles pass forward and upward to the crura cerebri and the 

 optic thalami. These connect the cerebellum wkh the cerebrum. Beneath 

 the tubercular quadrigemina, some of these fibres decussate with the corre- 

 sponding fibres from 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 cerebral hemisphere of the opposite side. The connections 

 between the cerebrum and the cerebellum, through the pons Varolii, have 

 already been described (see page 610). 



The middle peduncles arise from the lateral hemispheres of the cerebel- 

 lum, pass to the pons Varolii, where they cross, connecting the two sides of 

 the cerebellum. 



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 is sufficiently evident. By the superior peduncles, the cerebellum 

 is connected, as are all of the encephalic ganglia, with the cerebrum ; by the 

 middle peduncles, the two lateral halves of the cerebellum are intimately con- 

 nected with each other ; and by the inferior peduncles, the cerebellum is 

 connected with the posterior columns of the spinal cord. 



Extirpation of the Cerebellum. When the greatest part or the whole of 

 the cerebellum is removed from a bird or a mammal, the animal being, before 

 the operation, in a perfectly normal condition and no other parts being 

 injured, there are no phenomena constantly and invariably observed except 

 certain modifications of the voluntary movements (Flourens). The intelli- 

 gence, general and special sensibility, the involuntary 'movements and the 

 simple faculty of voluntary motion remain. The movements are always 

 exceedingly irregular and inco-ordinate ; the animal can not maintain its 

 equilibrium ; and on account of the impossibility of making regular move- 

 ments, it can not feed. This want of equilibrium and of the power of co-or- 



