382 THE NERVOUS SYSTEM. 



when the ulnar nerve, which supplies the fifth and a part of 

 the fourth finger, is divided, the sensibility of those parts is 

 not preserved through the medium of the branches which the 

 ulnar derives from the median nerve ; but the fourth and fifth 

 fingers are permanently deprived of sensibility. On the other 

 hand, there are instances in which the trunk of the chief sen- 

 sitive nerve supplied to a part having been divided, the sensi- 

 bility of the part is still preserved by intercommunicating 

 fibres from a neighboring nerve-trunk. Thus, a case is related 

 by Mr. Savory in which, after excision of a portion of the 

 musculo-spiral nerve, the sensibility of some of the parts sup- 

 plied by it, although impaired, was not altogether lost, prob- 

 ably on account of those fibres from the external cutaneous 

 nerve which are mingled with the radial branch of the mus- 

 culo-spiral. One of the uses of a nervous plexus (p. 372) is 

 here well illustrated. 



Several of the laws of action in motor nerves correspond 

 with the foregoing. Thus, the motor influence is propagated 

 only in the direction of the fibres going to the muscles ; by 

 irritation of a motor nerve, contractions are excited in all the 

 muscles supplied by the branches given off by the nerve 

 below the point irritated, and in those muscles alone : the 

 muscles supplied by the branches which come off from the 

 nerve at a higher point than that irritated, are never directly 

 excited to contraction. No contraction, for instance, is pro- 

 duced in the frontal muscle by irritating the branches of the 

 facial nerve that ramify upon the face ; because that muscle 

 derives its motor nerves from the trunk of the facial previous 

 to these branches. So, again, because the isolation of motor 

 nerve-fibres is as complete as that of sensitive ones, the irrita- 

 tion of a part of the fibres of the motor nerve does not affect 

 the motor power of the whole trunk, but only that of the por- 

 tion to which the stimulus is applied. And it is from the same 

 fact that, when a motor nerve enters a plexus and contributes 

 with other nerves to the formation of a nervous trunk pro- 

 ceeding from the plexus, it does not impart motor power to 

 the whole of that trunk, but only retains it isolated in the 

 fibres which form its continuation in the branches of that 

 trunk. 



Functions of Nerve- Centres. 



As already observed (p. 375), the term nerve-centre is applied 

 to all those parts of the nervous system which contain gan- 

 glion corpuscles, or vesicular nerve-substance, i. e., the brain, 

 spinal cord, and the several ganglia which belong to the cere- 

 bro-spinal and the sympathetic systems. Each of these nervous 



