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AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



a second maximum (Fig. 19, d) is reached, beyond which no further increase 

 is to be obtained, however much the current may be strengthened. 1 



Fig. 19.— Effect of increase of strength of current on the efficiency of breaking induction shocks (after 

 Fick) : a, minimal contraction ; b-c, first maximum ; d-e, second maximum. 



If both the making and breaking contractions be recorded, inasmuch as 

 the making shocks are weaker stimuli than the breaking (see p. 35), the mak- 

 ing contractions do not appear until after the breaking contractions have 

 acquired a considerable height. After the making minimal contraction has 

 been obtained, the making contractions rapidly gain in height as the current 

 is strengthened, and finally acquire the same height as the maximal breaking 

 shocks. 



The relation of the strength of the electric current to its irritating power 

 can be demonstrated equally well by using the direct galvanic current. The 

 strength of the galvanic current depends upon the character and number of the 

 cells employed, and the total resistance in the circuit. The strength of the 

 current can be easily varied by altering the resistance, and there are a number 

 of forms of apparatus for this purpose. 



Fig. 20.— Rheostat. 



A convenient instrument is the rheostat (Fig. 20). This is a box containing coils of 

 wire of known resistance. These coils are connected with a series of heavy brass blocks on top 

 of the box. The current enters the box by a binding-post attached to the first of the brass 

 blocks and passes thence from block to block, by going through the coils of wire connecting 

 them, until it reaches the binding-post at the other end of the series. The blocks can be 

 also connected by good conducting brass plugs, which can be pushed in between them, and 

 when this is done, as the current passes directly from block to block instead of going through 

 the resistance coils beneath, the resistance is reduced to a corresponding amount. 



Another method of altering the strength of the current flowing through the 

 nerve is to employ some form of shunt to split the current so that only a part 

 of it shall pass by way of the nerve. A current takes the path of least resist- 



1 Fick: Untersuchungen iiber elektrische Nervenreizung, Braunschweig, 1864. 



