THE NERVOUS SYSTEM 



309 



the line of the old fibre, become changed at first into a jelly- 

 like material and later into nerve-fibre substance. This new 

 nerve fibre makes connections with the stump of the old one, 

 and communication is again possible between the cell in the 

 cord or brain and the old nerve ending. 



Nerve Endings. As has been pointed out, nerves are divided 

 into two classes: (1) the afferent nerves, which bring in im- 

 pulses from the outside world or from internal organs to the 

 spinal cord or brain; and (2) the efferent nerves, which take 

 impulses out from the cord or brain to the organs which they 

 supply. 



Several kinds of afferent nerve endings, or more properly 

 beginnings, in the skin or elsewhere have already been re- 

 ferred to and shown in Figures 112, 117 and 1 18. In general, 

 they may be said to start in very minute spheres or oblong 

 bodies called corpuscles. The nerve fibrils begin here as fine 

 branches either on the ^ Nerve . 



exterior or in the in- 

 terior of these organs. 

 These skin end-organs 

 receive mainly impres- 

 sions of touch and tem- 

 perature, an extreme of 

 either taking the form 

 of pain. 



The efferent nerves 

 are distributed almost 

 entirely to muscles and 

 glands and excite these to action. The manner in which 

 nerves end in muscles is diagrammatically shown in Figure 

 162. It is not possible to say just what happens in a muscle 

 or gland when a message is delivered to it. The muscle 

 contracts or the gland secretes, but just what change in their 

 protoplasm excites these activities is not known. 



FIG. 162. THE ENDING OF A NERVE IN 



MUSCLE FIBRES 



The end organ of a motor nerve. (Bohn & 

 Davidoff) 



