102 THE CONTRACTILE TISSUES. 



Suppose that on the nerve of a muscle-nerve preparation are placed two 

 (non-polarizable) electrodes (Fig. 32, a, &), connected with a battery and 

 arranged with a key, so that a constant current can at pleasure be thrown 

 into or shut off from the nerve. This constant current, whose effects we are 

 about to study, may be called the " polarizing current." Let a be the posi- 

 tive electrode or anode, arid k the negative electrode or kathode, both placed 

 at some distance from the muscle, and also with a certain interval between 

 each other. At the point x let there be applied a pair of electrodes con- 

 nected with an induction-coil. Let the muscle further be connected with a 

 lever, so that its contractions can be recorded and their amount measured. 

 Before the polarizing current is thrown into the nerve, let a single induction- 

 shock of known intensity (a weak one being chosen, or, at least, not one 

 which would cause in the muscle a maximum contraction) be thrown in at 

 x. A contraction of a certain amount will follow. The contraction may 



FIG. 32. 



Muscle-nerve Preparations : with the nerve exposed in A to a descending and in B to an ascend- 

 ing constant current. In each, a is the anode, k the kathode of the constant current ; x represents 

 the spot where the induction-shocks, used to test the irritability of the nerve, are sent in. 



be taken as a measure of the irritability of the nerve at the point x. Now 

 let the polarizing current be thrown in and let the kathode or negative pole 

 be nearest the muscle, as in Fig. 32, A, so that the current passes along the 

 nerve in a direction from the central nervous system toward the muscle ; 

 such a current is spoken of as a descending one. The entrance of the polar- 

 izing current into the nerve will produce a " making" contraction ; this we 

 may neglect. If while the current is passing the same induction-shock as 

 before be sent through x, the contraction which results will be found to be 

 greater than on the former occasion. If the polarizing current be now shut 

 off, a "breaking" contraction will probably be produced; this we also may 

 neglect. If, now, the point x, after a short interval, be again tested with the 

 same induction-shock as before, the contraction will be no longer greater, 

 but of the same amount, or perhaps not so great as at first. During the 

 passage of the polarizing current, therefore, the irritability of the nerve at 

 the point x has been temporarily increased, since the same shock applied to 

 it causes a greater contraction during the presence than in the absence of the 

 current. But this is only true so long as the polarizing current is a descend- 

 ing one so long as the point x lies on the side of the kathode. On the other 

 hand, if the polarizing current had been an ascending one, with the anode or 



