8o HUMAN PHYSIOLOGY 



of automatic action on the part of any portion of the nervous 

 system. Throughout life these neurons are constantly being 

 acted upon from without, and activity, once started by any 

 stimulus, must necessarily set up a stream of action which 

 will be co-existent with life. 



2. Stimulation of Neurons. This implies that a neuron is 

 capable of stimulation, that, like all other protoplasm, it 

 reacts to changes in external conditions. A neuron may be 

 stimulated at any part, but it is usually stimulated from one 

 or other of its terminal dendritic endings, either by changes in 

 the tissues round about or by changes in other neurons. Thus 

 (Fig. 34) a neuron may be thrown into action by changes in 

 the tissue at its extremity, while another may be stimulated 

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

 in their course, as is demonstrated by pinching the ulnar 

 nerve behind the internal condyle of the humerus. 



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

 any sudden change excites it 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 neighbour- 

 hood, 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. 46 et seq.). 



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 the many fac- 

 tors. It may be increased by a slight cooling, but is de- 

 creased at lower temperatures. It is increased by a 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, de- 

 creased around the positive pole, in the same way as in muscle 

 (see p. 48). 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 nerve, and the 

 phenomena of fatigue are not manifested as in muscle. 



