ELECTROTONIC STATES OF NERVES. 509 



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 im- 

 pulse. 



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 anelectrotonus 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 decrease and increase of irritability become less marked 



