NERVE 



683 



injured nerve. For instance, experiments on the phrenic nerve, 

 in its natural position, and with all its connections intact, have 

 shown that with a given strength of stimulus the amount of 

 contraction of the diaphragm is the same whether the nerve be 

 excited in the upper, middle, or lower portion of its course. In 

 the above experiment on the isolated, and therefore injured, nerve, 

 the contraction varies in height with the distance of the point of 

 stimulation from the muscle, not because the excitation grows 

 as it travels, but because it is already greater at the moment 

 when it sets out from a point near the central end of the nerve 

 than at the moment when it sets out from a point near the 

 muscle. 



Electrotonus. Although the constant current does not, unless 

 it is very strong or the nerve very irritable, cause stimulation 



FIG. 253. KATELECTROTONUS. 



Weak tetanus of muscle (the 

 right-hand elevation), greatly in- 

 tensified in katelectrotonus of the 

 motor nerve (the left-hand eleva- 

 tion). 



FIG. 254. ANELECTROTONUS. 



Strong tetanus of muscle (left- 

 hand elevation), lessened in 

 strength by anelectrotonic con- 

 dition of the motor nerve (right- 

 hand elevation). 



during its passage, it modifies profoundly the excitability and 

 conductivity of the nerve. In the neighbourhood of the kathode 

 the excitability is increased (condition of katelectrotonus), while 

 around the anode it is diminished (anelectrotonus) . Immediately 

 after the opening of the current these relations are for a brief 

 time reversed, the excitability of the post-kathodic area (area 

 which was at the kathode during the flow) being diminished, and 

 that of the post-anodic increased. In the intrapolar area there 

 is one point the excitability of which is not altered. This in- 

 different point, as it is called, shifts its position when the intensity 

 of the current is varied, moving towards the kathode when the 

 current is increased, towards the anode when it is diminished. 



These statements have been made on the strength of experiments 

 in which the height of the muscular contraction was taken as the 

 index of the excitability of the nerve at any given point. But 



