THE NERVOUS SYSTEM OF MAN. 



57 



Every mechanism for action and reaction between 

 the organism and the external world must consist of 

 two kinds of central cells (a sensory and a motor), two 

 kinds of transmitting fibers (afferent and efferent), and 

 two kinds of peripheral terminations (a sensitive sur- 

 face or sense organ and a muscular or contractile tissue). 

 Fig. 36, repeated from page 26, is a diagram illustrating 

 this action and reaction. The manner in which the 

 whole acts is briefly as follows: Impression on a sen- 

 sitive surface or sense organ (S) is transmitted cen- 

 tripetally along a sensory fiber to the nerve center, 

 awakens response, which is transmitted centrifugally 



Fig. 36. — Diagram showing essential parts of an apparatus of exchange be- 

 tween the external world and consciousness : JVC, nerve center ; sc, sen- 

 sory cell ; sf, sensory fiber ; SS, sensory surface ; mc, motor cell ; mf, 

 motor fiber ; M, muscle. Arrowheads show the direction of transmis- 

 sion. 



along a motor fiber, and determines muscular contrac- 

 tion, which produces motion. 



Both these kinds of fibers are inclosed in the same 

 sheath in the case of spinal nerves, but are usually sepa- 

 rated and found in different nerves in the case of the 

 cranial nerves. Thus in the case of the cranial series 

 we speak of sensory and motor nerves, but in the spinal 

 series we can only speak of sensory and motor fibers. 



The Two Subsystems. — The cerebro-spinal system may 

 be conveniently subdivided into two subsystems. By 

 function they may be called the conscio-voluntary and the 



