440 STUDIES IN ADVANCED PHYSIOLOGY. 



continuation, and from this be carried to muscle or gland, 

 or else in some sense-organ to be carried inwards to the 

 appropriate center. This brings up the question, in what 

 manner a nerve may be stimulated to transmit such im-. 

 pulses. 



Nerve Stimuli. 



In the laboratory it is possible to stimulate a nerve arti- 

 ficially in the following ways : 



1. By Mechanical Stimuli. A tap, a pinch or blow on 

 a living exposed nerve excites it and is the occasion of an 

 impulse through the nerve. A familiar illustration of this 

 is found in striking the crazybone at the elbow, which is 

 in reality striking the nerve at this point. The blow occa- 

 sions an impulse which runs to the brain, and by the brain 

 is referred, although erroneously, to the fingers. 



2. Thermal Stimuli. Sudden change in the tempera- 

 ture excites the nerve, be the change upward or downward. 

 If this change, however, is very gradual, the nerve seems 

 to be able to accustom itself to the change and no direct 

 impulse arises. 



3. Chemical Stimuli. Quite a number of substances 

 chemically alter a nerve fiber and so stimulate it. Thus 

 immersing the end of a nerve in a strong solution of com- 

 mon salt excites it. Here, too, if such a nerve be placed in 

 a very dilute solution and this solution then gradually made 

 more concentrated no excitation follows. 



4. Electrical Stimuli. One of the most satisfactory 

 ways to stimulate a nerve in experimental physiology is by 

 means of the electrical current. An electric shock passing 

 through a nerve fiber at any part along its course power- 

 fully excites it. Even a sudden change in the strength of 

 the current through a nerve excites it. A steady current, 

 however, has no effect, but when such a current is suddenly 

 broken, the sudden disappearance of the ciirrent acts as a 

 stimulus. 



The explanation of all these stimuli lies in the siidden- 

 ness with which any change is brought about in the nerve 



