76 NERVOUS SYSTEM OF VERTEBRATES. 



CHAPTER IV. 

 NERVE ELEMENTS AND THEIR FUNCTIONS. 



The chief functions of nerve elements are the origination 

 and transmission of nerve impulses. A nerve impulse originates 

 as the result of a stimulus. Although the stimulus may be one 

 of several kinds, mechanical, chemical, thermal, photic, etc., it 

 always consists in some change in the environment of the nerve 

 cell. Whatever the character of the stimulus, it produces in the 

 protoplasm of the nerve cell chemical and physical changes of 

 such a character that they may be propagated or transmitted 

 through the protoplasm from one part of the nerve cell to another. 

 These changes within a nerve cell constitute a nerve impulse. 

 Although it is called forth by a stimulus, the impulse is not that 

 stimulus caught up and passed on, but is a new thing consisting in 

 the activity of the nerve cell itself. Besides the origination and 

 transmission of impulses it is believed that other functions may be 

 performed by nerve elements. They may reinforce or strengthen 

 the impulses during their transmission; they may store up weak 

 impulses so as to discharge a stronger one after an interval (sum- 

 mation) ; such discharge may be repeated at more or less regular 

 intervals (rhythmical nerve action) ; or the impulses may be more 

 or less completely blocked or impeded in their transmission 

 (inhibition). The consideration of these processes within nerve 

 elements belongs to the field of general nerve physiology with 

 which the present book does not deal. 



Although the chemical and physical nature of a nerve impulse 

 may and probably does differ in relation to the internal structure 

 of the nerve cell and the character of the stimulus, the importance 

 of the nerve impulse to the organism consists in the effect produced 

 when the nerve impulse is delivered by the nerve element to some 

 other tissue element. What tissue element shall receive the 

 impulse ; after how long a time it shall be delivered ; whether it shall 



