64 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



For convenience these four cases are represented by the abbreviations ACC, 

 AOC, KCC, and KOC. 



These cases may be arranged in order according to the strength of the 

 irritation which is developed. 



Since the irritation process developed at a physiological kathode by 

 closing a current, is, other things being equal, stronger than that developed 

 at a physiological anode by opening the current, we should expect that the 

 two closing contractions, KCC and ACC, would be stronger than the two 

 opening contractions, KOC and AOC. This is the case, and as the current is 

 more dense in the region of the physiological kathode, beneath the physical 

 kathode, than at the physiological kathode, beneath the physical anode, KCC 

 is stronger than ACC. 



Of the two opening contractions, AOC is stronger than KOC because 

 of the greater density of the current in the region of the physiological anode, 

 beneath the physical anode, than in the region of the physiological anode, 

 beneath the physical kathode. 



These differences in the strength of the irritation process developed in these 

 different regions is well shown by examining the reaction of nerves to cur- 

 rents of gradually increasing strength. The effect of the opening and closing 

 irritation is seen to be as follows : 



Weak currents. Medium currents. Strong currents. 



KCC KCC KCC 



ACC ACC 



AOC AOC 



KOC 



The natural order, therefore, would be KCC, ACC, AOC, KOC. Some- 

 times, however, AOC is stronger than ACC ; this happens when on account 

 of the relation of the surrounding tissues to the nerve the density of the cur- 

 rent at the physiological anode is great as compared with the density at the 

 physiological kathode. 



When the currents employed are strong, it not infrequently happens in the 

 case of men that not only are the make and break followed by the usual rapid 

 contractions of short duration, but during the closure of the current there is 

 ^continued contraction galvanotonus, as it is sometimes called. 



Conditions which Determine the Irritability of Nerves and Muscles. 

 We have thus far considered the conditions which determine the efficiency 

 of such an irritant as the electric current. Other irritants are subject to like 

 conditions, their activity being controlled to a considerable extent by the sud- 

 denness, strength, density, duration, and, possibly, direction of application. It 

 is not necessary for us to consider how each special form of irritant is affected 

 by these conditions ; it will be more instructive for us to study how different 

 irritants alter the irritability of nerve and muscle, and the relation of irri- 

 tability to the state of excitation. 



The power to irritate is intimately connected with the power to heighten 



