INTRODUCTION. 



11 



these central parts is broken. The true cause of 

 movement resides in these parts. A sort of change, 

 the essential nature of which is unknown to us, takes 

 place in them, and is propagated along the nerve to 

 the muscle, causing it to contract. The central ner- 

 vous system is, therefore, the origin, the centre from 

 which the order to contract proceeds ; hence its name. 

 The nerves which run from these central parts to the 

 muscles are known as the nerves of movement (motor 

 nerves). 



There is still, however, a second group of nerves, 

 the nerves of sensation (sensory nerves), which arise 

 in the sense-organs (skin, mucous membrane of tongue, 

 nose, ear, eye), and convey to the central nervous 

 system the impressions they receive from the outer 

 world by the aid of these sense-organs. In the ap- 

 pended diagram (Fig. 5), G represents the central 



Fig. 5 — Diagram to explain the Action of the Motor and Sensory Kerves. 



nervous system ; B.H., a motor nerve, branching in the 

 muscle M ; ff.i\^., a sensory nerve, which runs from the 

 blood-bathed inner skin or dermis (L.h), underlying 

 the outer skin or epidermis (O.h), to the central 

 system. (The arrows indicate the direction in which 

 impulses are conveyed along the corresponding nerve. 



