ELEOTROTONIC STATES OF NERVES. 507 



evidenced by the response given by the attached muscle to weak 

 stimuli. The more important of these are : 



1. Temperature changes. In the case of a frog's nerve, a rise 

 of temperature to 30-40 C. causes an increase in its excitability. 

 Also a fall of temperature below zero tends to make the nerve 

 more easily excited. Both these conditions have, however, a very 

 fleeting effect, for the nerve soon dies at the temperature named, 

 and most probably the increased irritability is only to be taken 

 as a sign of approaching death. It thus appears that a medium 

 temperature is the optimum for nerve work. 



2. The part of the nerve stimulated, is also said to have some 

 effect on the result of a given strength of stimulus. The further 

 from the muscle, the more powerful the contraction produced, 

 other things being equal. So that the impulse is supposed to 

 gather force as it goes, as in the case of a falling body, and hence 

 has been spoken of as the avalanche action of nerve impulse. 



3. A new section of the nerve is said to increase its irritability, 

 as does, indeed, any slightly stimulating influence, such as. dry- 

 ing, and chemical or mechanical meddling of any kind. This 

 increase in irritability probably depends upon injurious changes 

 going on in the nerve, as the influences just alluded to lead to 

 complete loss of excitability if carried too far. 



4. The most remarkable change in the excitability of a nerve, 

 is that brought about by the action of a constant current passing 

 through the nerve, so as to set up the conditions just described 

 as auelectrotonus and catelectrotonus. (a) The irritability of the 

 nerve is considerably increased in the region near the cathode, 

 and it is notably diminished in the neighborhood of the anode. 



(/?) The increase of irritability is in proportion to the intensity 

 of the catelectrotonic state, and the decrease in proportion to the 

 intensity of the anelectrotonus. Thus the increase is most marked 

 in the immediate neighborhood of the cathode, and fades with 

 *the distance from the negative pole ; and similarly, the decrease 

 is strongest at the anode, and become less and less as it passes 

 away from the positive pole. In the same way, in the part of the 

 nerve between the two poles the intrapolar region the de- 

 crease and increase of irritability become less marked towards the 



