GENERAL NERVE PHYSIOLOGY 



221 



(1) During making, when the anode lies between the 

 end-organ and the kathode. 



(2) During breaking, when the kathode lies between the 

 end-organ and the anode. 



Law of contraction. From the above-mentioned facts the 

 following relations between the stimulation of a muscle and 

 the strength of a current and its direction through the motor 

 nerve can be formulated : 



The current is ascending when it passes through the nerve from 

 the muscle to the centre; it is descending when it passes from the 

 centre to the muscle. 



The law of contraction may be explained thus : In a weak cur- 

 rent only a stronger stimulus is active, i.e. the appearance of 

 katelectrotonus stimulates; not so the disappearance of anelectro- 

 tonus, hence a contraction occurs only by the making. In a 

 moderate current both the appearance of katelectrotonus and the 

 disappearance of anelectrotonus stimulate, hence both a make and 

 a break contraction result. 



In a strong ascending current the make contraction fails, 

 because the stimulation at the kathode cannot pass through that 

 part of the nerve where the conductivity has been decreased at the 

 anode. In a strong descending current the break contraction 

 fails, because the stimulation at the anode cannot pass through 

 the non-conducting part of the nerve at the kathode. 



A current passing through a stretch of medullated nerve spreads 

 throughout the whole nerve; also to that part of the nerve beyond 

 the electrodes. If the two electrodes of a galvanometer are placed 

 upon the nerve exterior to the part stimulated, a current passes 

 through the galvanometer because of this spreading. This 

 phenomenon is of physiological interest, for this spreading is due 

 to a peculiar polarization of living nerve fibres. This spreading 

 of the current cannot be demonstrated in a dead nerve. The 



