GENERAL MUSCLE PHYSIOLOGY 197 



4. Electrical. A constant current of sufficient strength 

 passing through a muscle in its longitudinal direction causes 

 contraction when the current is made, and sometimes also 

 when the current is broken. During the passage of the 

 current through the muscle, the muscle undergoes a lasting 

 contraction, which is, however, less marked than the initial 

 or make contraction. 



If the current is passed transversely through the muscle, 

 it does not stimulate it. 



By studying the nature of the wave of contraction, it has 

 been found that in the make contraction the stimulation 

 begins at the negative pole (kathode) and from there spreads 

 throughout the muscle, while in the break contraction it 

 begins at the anode. 



Induction currents stimulate only at the kathode. 



In order to stimulate, the current must be active for a 

 certain length of time ; currents lasting for a very short time 

 are not effective. The different kinds of muscles behave 

 differently in this respect . While striated muscles are more 

 affected by sudden changes in the intensity than by long 

 duration of the current, in case of smooth muscles it is the 

 reverse. 



At the places of entrance and exit the current changes 

 the irritability of the muscle in the same manner as in the 

 nerve (see page 220). 



The electromotor resistance of the muscle in the longitudinal 

 direction is two and one-half million, that in the transverse direc- 

 tion is twelve and one-half million, times greater than that of 

 mercury. 



Relation between the stimulation and the contraction. The 

 extent of the contraction (measured by the heat produced) 

 increases with the strength of stimulation up to the higher 

 limit, beyond which increase in the stimulation does not 

 produce anjncrease in contraction. 



The irritability of the muscle is dependent upon the 

 normal vital processes, as well as upon the previously 

 mentioned influences (mechanical conditions, temperature, 

 chemical agents, electrical currents). 



