538 THE NERVOUS SYSTEM 



ing its cell body, takes place. This happens particularly in those relations 

 where the original neurone forms a link in a conducting path. Such a con- 

 ducting neurone would no longer perform its proper function so would 

 atrophy just as would a muscle fiber when cut off from its nervous relations. 



In conclusion, one may infer that the cell body exercises a nutritive or 

 trophic control over the protoplasm of its branches, just as we have already 

 seen the neurone as a whole exercises trophic control over the nutritive 

 processes taking place in the tissue to which its branches are distributed, for 

 example, the voluntary muscles. 



Specific Energy of the Nerve Impulses. We have already discussed 

 the fact that a nerve fiber, also its cell body, is irritable to various forms of 

 mechanical, electrical, and other stimuli. In the complex of activity of the 

 nervous system it is found that whatever the form of the external stimulus 

 applied to a nerve, the resulting nerve impulse produces the same effects 

 in the central nervous system. The reaction in consciousness is constant 

 and unvarying and independent of the character of the external stimulus. 

 For example, if the temperature points on the skin be stimulated, as they 

 may, by a number of widely different types of external stimulus, the result- 

 ing sensation is the same; stimulation of a "heat point" by ice produces a 

 sensation of heat, not cold. This phenomenon has been called the specific 

 energy of the nerve impulse, and the term was first advanced by Johannes 

 Miiller. Different views are presented in explanation. But it seems ra- 

 tional to believe that the gist of the matter rests in two factors: i. The 

 highly differentiated sense organ is adapted especially to stimulation by a 

 particular stimulus, as the eye by light. 2. The central apparatus is de- 

 veloped in response to and especially adapted to receive the specific stimulus. 

 The interpretations that are made in consciousness in response to an inflow 

 of nerve impulses from the sense organ are more or less constant. When 

 the exceptional stimulus is applied to the special end organ it results in the 

 usual change in the sense organ, the resulting nerve impulses reach the usual 

 central area, and there is no physiological basis for other than the usual sen- 

 sations which development and experience have associated with activity in 

 the parts affected. 



Transmission of Nerve Impulses through the Neurone. The the- 

 ory has been advanced that in the nerve cell the primary function of some 

 processes is to carry nerve impulses toward the cell body, and of other proc- 

 esses to carry nerve impulses away from the cell body. At the present time 

 this view is advocated by perhaps the ablest living anatomists and neurolo- 

 gists. The dendrites conduct toward the cell body, and the axones away from 

 it. That is, the former are cellulipetal, the latter cellulifugal. 



Impressions made upon the terminations or upon the trunk of an afferent 

 nerve may cause, a, pain or some other kind of general sensation; b, special 

 sensation; c, reflex action of some kind; or d, inhibition or restraint of action. 



