TRANSMISSION OF NERVE IMPULSES THROUGH THE NEURONE 577 



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

 may, by a number of widely different types of external stimuli, 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; fc, special 

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

 Similarly impressions made upon an efferent nerve may cause, a, contraction 

 of muscle (motor nerve); b, it may influence secretion (secretory nerve); 

 c, it may influence nutrition (trophic nerve) ; or d, it may inhibit, augment, 

 or stop any other efferent action. 



By artificial stimulation nerve impulses can be made to pass in both di- 

 rections in all classes of nerve processes. That is to say, if a motor axone is 

 artificially stimulated in the middle of its course it will not only convey a nerve 

 impulse to its distribution, but also a nerve impulse will pass back over the fiber 

 to the cell body and out over the dendrites. Normally, in the complex of the 

 body, it is probable that such a neurone will be stimulated only at its points 

 of contact with other neurones chiefly through its dendrites, and especially 

 by means of the sensory cells. The dendrites therefore will receive the nerve 

 stimulus, carry it through the cell body to the axone and its distribution. 

 In such cells there is isolated, or uninterrupted, conduction throughout the 

 extent of the neurone. The nerve impulse is able to pass from a given 

 neurone to adjacent ones only at the termination of the axone or its branches, 



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