546 THE NERYOUS SYSTEM. 



The sensitive function of the branches of the greater 

 division of the fifth nerve is proved by all the usual evi- 

 dences, such as their distribution in parts that are sensitive 

 and not capable of muscular contraction, the exceeding 

 sensibility of some of these parts, their loss of sensation 

 when the nerve is paralyzed or divided, the pain without 

 convulsions produced by morbid or experimental irritation 

 of the trunk or branches of the nerve, and the analogy of 

 this portion of the fifth to the posterior root of the spinal 

 nerve. 



But although formed of sensitive filaments exclusively, 

 the branches of the greater or ganglionic portion of the 

 fifth nerve exercise a manifold influence on the movements 

 of the muscles of the head and face, and other parts in 

 which they are distributed. They do so, in the first place, 

 by providing the muscles themselves with that sensibility 

 without which the mind, being unconscious of their position 

 and state, cannot voluntarily exercise them. It is, pro- 

 bably, for conferring this sensibility on the muscles, that 

 the branches of the fifth nerve communicate so frequently 

 with those of the facial and hypoglossal, and the nerves of 

 the muscles of the eye ; and it is because of the loss of this 

 sensibility that when the fifth nerve is divided, animals are 

 always slow and awkward in the movement of the muscles 

 of the face and head, or hold them still, or guide their 

 movements by the sight of the objects towards which they 

 wish to move. 



Again, the fifth nerve has an indirect influence on the 

 muscular movements, by conveying sensations of the state 

 and position of the skin and other parts : which the mind 

 perceiving, is enabled to determine appropriate acts. Thus, 

 when the fifth nerve or its infra- orbital branch is divided, 

 the movements of the lips in feeding may cease, or be im- 

 perfect ; a fact which led Sir Charles Bell into one of the 

 very few errors of his physiology of the nerves. He sup- 

 posed that the motion of the upper lip, in grasping food, 



