78 VETERINARY PHYSIOLOGY 



1. Stimulation of Neurons. Neurons, like all other 

 protoplasm, react to changes in external conditions ; they 

 are capable of stimulation. A neuron is usually stimulated 

 from one or other of its terminal dendritic endings, either by 

 changes in the tissues round these or by changes in other 

 neurons. Thus (fig. 35) a neuron may be thrown into action 

 by changes in the tissue at its extremity, and a second may be 

 stimulated by the activity of the former. They may also be 

 stimulated at any part of their course, as may be demonstrated 

 by pinching the ulnar nerve behind the internal condyle 

 of the humerus. 



Means of Stimulation. Just as with muscle, so with neurons ; 

 any sudden change excites to activity be this change a 

 mechanical one, as in pinching a nerve, or a change in the 

 temperature, or in the electric conditions in its neighbourhood, 

 or in the chemical surroundings of the neuron agents which 

 withdraw water, like glycerine, stimulating most strongly. 

 All that has been said of the stimulation of muscle applies 

 to the stimulation of nerve (see p. 42 et seq.). practical 

 Physiology.) 



The condition of the neuron modifies the effect of the 

 stimulus, and the condition of other neurons modifies the 

 ultimate result of the stimulus on the body. 



The excitability of a neuron is modified by many factors. 

 It may be increased by a slight cooling, but is decreased at 

 lower temperatures. It is increased by warming up to a certain 

 point. Drying at first increases excitability, then abolishes 

 it. During the flow of an electric current it is increased 

 in the neighbourhood of the negative pole, decreased around 

 the positive pole, in the same way as in muscle (see p. 42). 

 It is influenced by many chemical substances, some of which 

 increase its excitability in small doses, and diminish it in 

 larger doses ; some again even in the smallest dose depress its 

 activity, e.g. potassium salts. Continued activity has no effect 

 on the . excitability of axons, and the phenomena of fatigue are 

 not manifested in them. 



2. Manifestations of the Activity of Neurons. So far as is at 

 present known, the activity of neurons is not accompanied by 

 any obvious change in them, although it is possible that 



