CHAPTER XVIII 

 PHYSIOLOGY OF THE NERVOUS SYSTEM 



IN the preceding chapter was pointed out the method 

 by which the different parts of the body are brought into 

 communication by the neurons or nerve cells. We are 

 now to study the means whereby the neurons are made 

 to control and coordinate the different parts of the body 

 and bring about the necessary adjustment of the body to 

 its surroundings. This work of the neurons naturally has 

 some relation to their properties. 



Properties of Neurons. The work of the neurons seems 

 to depend mainly upon two properties the property of 

 irritability and the property of conductivity. Irritability 

 was explained, in the study of the muscles (page 243), as 

 the ability to respond to a stimulus. It has the same 

 meaning here. The neurons, however, respond more 

 readily to stimuli than do the muscles and are therefore 

 more irritable. Moreover, they are stimulated by all the 

 forces that induce muscular contraction and by many 

 others besides. They are by far the most irritable por- 

 tions of the body. 



Conductivity is the property which enables the effect of 

 a stimulus to be transferred from one part of a neuron 

 to another. On account of this property, an excitation, 

 or disturbance, in any part of a neuron is conducted or 

 carried to all the other parts. Thus a disturbance at the 

 distant ends of the dendrites causes a movement toward 

 the cell-body and, reaching the cell-body, the disturbance is 



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