132 NERVOUS SYSTEM. 



nerves, for sensibility is not affected by its injury or disease. 

 The motor system is, however, entirely disorganized by lesions 

 of the organ. Co-ordination of the voluntary muscles is accom- 

 plished by this portion of the brain, and it is originated in the gray 

 matter of the part. It has no effect upon the senses or upon the 

 intellect, so far as is known. 



What is the effect of removing the cerebellum in animals ? 



When small portions are removed the animals become feeble and 

 uncertain in their movements, but are able to move for ordinary 

 purposes. As the amount removed increases the want of co-ordi- 

 nation of the voluntary muscles increases. With the entire cere- 

 bellum gone the condition is absolute animals cannot stand or 

 walk or bring any of the muscles into orderly action. If the ani- 

 mal is laid upon the back, it cannot recover itself, but struggles 

 vaguely in the attempt. The senses are apparently normal and the 

 will-power is present : if a blow is threatened, an attempt is made 

 to avoid it. When the lesion is confined to one hemisphere, the 

 lack of co-ordination is noticed in the opposite half of the body. 

 Under these circumstances the animals are apt to fall to the oppo- 

 site side and roll over and over rapidly. Such movements are 

 known as forced movements. This condition may persist for several 

 days. Pigeons from which the cerebellum is removed may live for 

 a considerable time, sometimes for several months, after the opera- 

 tion. In some cases there is a return of power to co-ordinate, after 

 partial removal, at the end of some days. 



THE CRANIAL, NERVES. 

 What are the cranial nerves? 



They are a set of twelve pairs of nerves which arise from the 

 brain. They are varied in their functions, but all arise from gan- 

 glia of gray matter in the brain and medulla. The floor of the 

 fourth ventricle is particularly rich in nuclei in which these cranial 

 nerves originate. 



How may the cranial nerves be classified? 



1. In the order of their emergence, by numbers : (I) Olfactory ; 

 (II) Optic; (III) Motor oculi; (IV) Patheticus; (V) Trigeminus; 

 (VI) Abducens ; (VII) Facial ; (VIII) Auditory ; (IX) Glosso- 

 pharyngeal ; (X) Pneumogastric ; (XI) Spinal accessory ; (XII) 



