428 THE FUNCTIONS OF CROSS-STRIATED MUSCLES 



of different size, a large one (12 X 6 cm.) applied to the breast, and a small one 

 (0.5-2 cm. diameter) applied over the motor point to be tested. Suppose now 

 the large electrode is the anode: the current enters then with relatively low 

 density, spreads out through the body with still less density and finally collects 

 at the cathode with great density. Since now the effects of a current depend 

 upon its density, it follows that with currents of moderate strength these effects 

 will appear only at the smaller electrode. Some of the many threads of current 

 reaching the smaller electrode from all parts of the body, will necessarily pass 



FIG. 165. Schematic representation of the entrance into and exit from a nerve of a current 

 applied to the skin over the nerve, after de Watteville. 



through the nerve under it. The effective cathode of the current lies where these 

 threads pass out of the nerve, and if, as we have assumed, the smaller electrode 

 is the cathode, other things being equal, the current will have its greatest possi- 

 ble density there. If the current is reversed so that it now enters the body by the 

 smaller electrode (which is still over the nerve), the places where the threads of 

 current leave the nerve constitute as before the effective cathode; the density 

 of the current now however is less than in the first case (Fig. 165). 



The polar law of excitation applies also to muscle, both with the constant 

 and induction current (v. Bezold, Engelmann, Biedermann; cf. page 416). 



We have a very instructive proof of this in the " polar failure " of excita- 

 tion discovered by Biedermann and Engelmann. If, for example, the end of a 

 frog's sartorius muscle be narcotized and the cathode be applied to this injured 

 place, on closing the current the muscle remains at rest. The normal muscle 

 substance is not stimulated by the closure of a current as it passes from the 

 normal to the paralyzed or dead muscle substance, and the mere passage of a 

 current is not sufficient to discharge the contraction (Locke and Szymanowski). 

 Similar phenomena may be shown on opening of the current when the anode 

 is placed at the injured place. 



E. EFFECT OF A RAPID SERIES OF STIMULI 



If a nerve or a muscle be affected by two stimuli in rapid succession, so 

 that the action resulting from the first has not yet come to an end when the 

 second becomes effective, the relaxation which would otherwise follow the 

 first contraction is interrupted and the effect of the second stimulus is added 

 to the first; consequently the contraction of the muscle is greater than it 

 commonly would be as the result of a single stimulus. It is only when the 

 load of the muscle is very light that it contracts as strongly to a single stimulus 

 as to rapidly repeated stimuli (v. Frey). 



