loo TEXT-BOOK OF PHYSIOLOGY. 



the nerve-fibers and the surface. They are especially adapted for the 

 reception of special stimuli and for the liberation of energy, which in turn 

 excites the nerve-fiber to activity. 

 General stimuli: 



1. Mechanic: Sharp taps, sudden pressure, cutting, etc. 



2. Thermic: Sudden application of heated object. 



3. Chemic: Contact of various substances which alter their chemic composi- 



tion quickly, e.g., strong acids or alkalies, sol. sodium chlorid 15 per 

 cent., sugar, urea, etc. 



4. Electric: Either the constant or induced current. 

 Special stimuli: 



For afferent nerves 



1. Light or ethereal vibrations acting upon the end-organs of the optic 



nerve in the retina. 



2. Sound or atmospheric undulations acting upon the end-organs of the 



auditory nerve. 



3. Heat or vibrations of the air acting upon the end-organs in the skin. 



4. Chemic agencies acting upon the end-organs of the olfactory and gusta- 



tory nerves. 



For efferent nerves 



A molecular disturbance in the central nerve-cells from which they arise, 

 the nature of which is unknown. 



Nature of the Nerve Impulse. As to the nature of the nerve impulse 

 generated by any of the foregoing stimuli, either general or special, but little 

 is known. It has been supposed to partake of the nature of a molecular 

 disturbance, a combination of physical and chemic processes attended by 

 the liberation of energy, which propagates itself from molecule to molecule. 

 The passage of the nerve impulse is accompanied by changes of electric 

 tension, the extent of which is an indication of the intensity of the molecular 

 disturbance. Judging from the deflections of the galvanometer needle it is 

 probable that when the nerve impulse makes its appearance at any given 

 point it is at first feeble, but soon reaches a maximum development, after 

 which it speedily declines and disappears. It may, therefore, be graphically 

 represented as a wave-like movement with a definite length and time dura- 

 tion. (See page 104.) Under strictly physiologic conditions the nerve 

 impulse passes in one direction only; in efferent nerves from the center to the 

 periphery, in afferent nerves from the periphery to the center. Experimen- 

 tally, however, it can be demonstrated that when a nerve impulse is aroused 

 in the course of a nerve by an adequate stimulus it travels equally well in 

 both directions from the point of stimulation. When once started, the 

 impulse is confined to the single fiber and does not diffuse itself to fibers 

 adjacent to it in the same nerve-trunk. 



Rapidity of Conduction of the Nerve Impulse. The passage of a 

 nerve impulse, either from the brain to the periphery or in the reverse direc- 

 tion, requires an appreciable period of time. The velocity with which the 

 impulse travels in human sensory nerves has been estimated at about 50 

 meters a second, and for motor nerves at from 28 to 33 meters a second. The 

 rate of movement is, however, somewhat modified by temperature, cold 

 lessening and heat increasing the rapidity; it is also modified by electric 



