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HUMAN PHYSIOLOGY. 



Special stimuli: 



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 nerye. 



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



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

 gustatory nerves. 



Nature of the Nerve Impulse. As to the nature of the nerve im- 

 pulse 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 

 chemical processes attended by the liberation of energy, which propa- 

 gates itself from molecule to molecule. Judging from the deflections 

 of the galvanometer needle it is probable that when the nerve im- 

 pulse 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 repre- 

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

 tion. Under strictly physiological 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. 

 Experimentally, 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 stimula- 

 tion. 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 Transmission of Nerve Force. The passage of a 

 nervous impulse, either from the brain to the periphery or in the 

 reverse direction, requires an appreciable period of time. The 

 velocity with which the impulse travels in human sensor nerves 

 has been estimated at about 190 feet a second, and for motor nerves 

 at from 100 to 200 feet a second. The rate of movement is, however, 

 somewhat modified by temperature, cold lessening and heat increas- 

 ing the rapidity ; it is also modified by electric conditions, by the 

 action of drugs, the strength of the stimulus, etc. The rate of trans- 

 mission through the spinal cord is considerably slower than in nerves, 

 the average velocity for voluntary motor impulses being only 33 feet 



