GENERAL STRUCTURE OF THE BODY. 



37 



they form the nerve fibers and carry impulses away from 

 the nerve cells. The cells of the processes are elongated 

 in shape, have a nucleus, and are placed end to end, with 

 a definite constriction between them. 



Each axis-cylinder process is surrounded by a sheath 

 called the medullary sheath, while each 

 nerve fiber consists of a central axis-cylin- 

 der process surrounded by the white sub- 

 stance of Schwann and enclosed in a 

 sheath. A bundle of these fibers invested 

 in a fibro-areolar membrane called the 

 neurilemma constitutes a nerve, and of 

 these the white matter is formed. The 

 blood supply is brought by minute vessels, 

 the vasa nervorum. 



The nerves of the cerebro-spinal system 

 preside over animal life and have to do 

 with voluntary acts, while those from the 

 sympathetic system regulate organic life 

 and are quite independent of the will. 

 Both sensory and motor nerves extend all 

 over the body, accompanying the arteries 

 in a general way. The sensory nerves 

 end on the surface in plexuses, in end 

 bulbs situated in the papillae of the skin, 

 or in tactile corpuscles, these last occur- 

 ring more especially where there is no medullary sheath 



, c, neurilemma; a, 



hair. The motor nerves end peripherally nucleus; e, node of 



! i i i , mi 1 Ranvier. (Leroy.) 



in plexuses or by end plates. The central 

 terminations of the motor nerves and the terminations 

 of sensory nerves in special organs, except where they 

 end in a cell, are not well understood. 



Like muscles, nerves are probably never at rest, for 

 through them the muscles get their tone. When a nerve 

 acts, no heat is produced and there is no change in the 

 nerve afterward, as there is in muscle. Probably nerve 

 impulse is the transmission of physical rather than chem- 

 ical changes along the fiber, the atoms of the nerve being 



