216 HUMAN PHYSIOLOGY 



petally, i.e. to their cells; the axis-cylinders carry it cellu- 

 lifugally, or from their cells. Accordingly, the peripheral 

 sensory nerves which carry the stimuli cellulipetally must 

 be regarded not as axis-cylinder processes but as elongated 

 protoplasmic processes. 



The process of irritability and conductivity of the indi- 

 vidual neuron is the elementary physiological process which 

 lies at the basis of the functions of the nervous system. 



The transmission of the stimulation from one neuron to another 

 is perhaps accomplished by delicate nerve fibnllae which connect 

 the end-tufts of one neuron with the protoplasmic processes of 

 another neuron. But it is difficult to demonstrate such fibrillae 

 anatomically. 



Neuroglia and medullary sheaths appear to be supporting and 

 protecting organs for the real nerve-substance. 



General nerve physiology is divided into two parts, corre- 

 sponding to the two parts of the neuron. 



1. General physiology of the nerve fibres, including the 

 sensory nerves which must really be regarded as dendrites. 



2. General physiology of the nerve cells. 



2. GENERAL PHYSIOLOGY OF THE NERVE FIBRES 



i. The irritability and conductivity of nerves, The 

 nerve fibres serve to carry impulses from one end-organ, the 

 receiving organ (sense organ or nerve cell), to the other 

 end-organ (muscle, gland cell, or other nerve cell). The 

 stimulation of a nerve takes place normally in the receiving 

 organ, but can also be applied to any part of the nerve by 

 artificial stimulation. 



The nature of the impulse and of the conduction of the 

 stimulation is not known. The only token of the impulse 

 which has been observed is an electrical phenomenon. An 

 active part of a nerve is negative to the resting part. The 

 significance of this phenomenon is not known. 



In Fig. 17 let AB be a nerve; at a and b place the electrodes 

 of a galvanometer (Z), and stimulate the nerve at Cby means of 

 an induction current. Shortly after stimulation the impulse 



