72 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



anode upon the removal of the current continues a considerable time and is 

 not reconverted to a decrease ; therefore the total after-effect is an increase of 

 irritability. 



The fact that when the current is closed the irritation starts from the kathode, 

 and when the current is opened from the anode, may well be associated with the 

 changes in irritability which take place at the kathode and anode upon the closing 

 and the opening of the current. The setting free of an irritation appears to be 

 associated only with an increase of irritability. When the current is closed the 

 establishment of the condition of katelectrotonus is accompanied by a rise of 

 irritability at the kathode, and when the current is opened the cessation of the 

 condition of anelectrotonus is likewise accompanied by a rise of irritability. In 

 the first case the irritability rises from the normal to something above the 

 normal, and in the second case the irritability rises from the condition of 

 decreased irritability up to something above the normal irritability. The change 

 from the normal to the anelectrotonic condition of decreased irritability, or 

 from the katelectrotonic condition of increased irritability down to normal 

 irritability, does not irritate. As has often been said, it is hard to distinguish 

 between increase of irritability and irritation. 



The effects produced by battery currents upon irritability are found to be 

 associated with peculiar alterations in the electrical condition of nerves and 

 muscles. The relation is a suggestive one, but cannot be taken as a definite 

 explanation of the changes of irritability. 



Effect of Frequency of Application of the Stimulus on Irritability. We have 

 seen that influences which act as irritants may also have an effect upon the irri- 

 tability of the nerve or muscle. In order to produce this change they must be 

 as a rule powerful, or act for a considerable time. Nevertheless, in the case 

 of muscles, at least, even a weak irritant of short duration, if repeated fre- 

 quently, tends to heighten irritability. For example, if a muscle be stimulated 

 by separate weak induction shocks at long intervals, the effect of each shock is 

 slight, and the change produced by it is compensated for by restorative pro- 

 cesses which occur within the living protoplasm during the following interval 

 of rest, and each of the succeeding irritations finds the mechanism in much the 

 same condition ; if, however, the shocks follow each other rapidly, each stimu- 

 lation leaves an after-effect which may have an influence upon the effectiveness 

 of the stimulus following it. As a result of this, induction shocks too feeble to 

 excite contractions may, if frequently repeated, after, a little time cause a visible 

 movement, and shocks of medium strength, if given at short intervals, may 

 each cause a larger contraction than its predecessor, until a certain height of 

 contraction has been reached, beyond which there is no further increase pos- 

 sible. It is not known whether the irritability of nerves is similarly increased, 

 nor is it known whether physiological stimuli exert such an influence. We 

 shall consider these so-called " staircase contractions " more carefully later (see 

 page 110). When irritations follow each other very rapidly, the whole cha- 

 racter of the contraction is changed, and the muscle, instead of making rapid 

 single contractions, enters into the condition of apparently continuous contrac- 



