508 MANUAL OF PHYSIOLOGY. 



currents, prevents the electrotonic current passing to the part of 

 the nerve which is thus separated. 



The condition of the portion of the nerve near the anode 

 positive pole is found to differ somewhat from that near the 

 cathode negative pole and hence it is found convenient to 

 speak of the region of the anode being in the aneledrotonic, and 

 that near the cathode being in the catelectrotonie condition. A 

 certain time is required for the production of anelectrotonus and 

 catelectrotonus, a current of less duration than .0015 of a second 

 being unable to bring about the change ; the negative variation 

 is, therefore, over before the electrotonus has commenced. 



IRRITABILITY OF NERVE FIBRES. 



The irritability of nerves varies according to certain conditions 

 and circumstances. While uninjured in the body, the irritability 

 of a nerve depends upon 



1. A perfect supply of blood so as to bear to it the necessary 

 quantity of new material as nutriment, and to carry off any injuri- 

 ous effete matters that may be produced by its molecular changes. 



2. A suitable amount of rest. Prolonged activity causes fatigue 

 and loss of irritability, no doubt from the same causes as were 

 mentioned as bringing about fatigue in muscles. The chemical 

 changes taking place in nerves, however, have not yet been made 

 out with any degree of accuracy. 



3. Uninjured connection with the nerve centres. When a spinal 

 nerve is cut, the part connected with the periphery rapidly under- 

 goes degenerative changes which seem to depend upon faulty 

 nutrition, since they are accompanied by structural changes 

 fatty degeneration. This appears to commence in a very short 

 time after the section commonly in about five days. The part 

 of the nerve remaining in direct connection with the cord retains 

 its irritability for a very much longer time. 



In the artificial stimulation, by means of electric shocks applied 

 to the nerve of a cold-blooded animal, there are many minor con- 

 ditions which have considerable influence on the irritability, as 

 evidenced by the response given by the attached muscle to weak 

 stimuli. The more important of these are : 



