. A text-book of human physiology . and cathodeswould be present at almost every possible point along the nerve, as indicatedby the radiating lines. Evidently such an experiment would not be adapted tothe study of electrical effects on human nerves. The monopolar method is there-fore used, the current being conveyed to and away from the body by electrodes 428 THE FUNCTIONS OP^ CROSS-STRIATED MUSCLES of different size, a large one (12 X G em.) applied to the breast, and a small one( cm. diameter) applied over the motor point to be tested. Suppose nowthe large electrode is the anode: the cu
. A text-book of human physiology . and cathodeswould be present at almost every possible point along the nerve, as indicatedby the radiating lines. Evidently such an experiment would not be adapted tothe study of electrical effects on human nerves. The monopolar method is there-fore used, the current being conveyed to and away from the body by electrodes 428 THE FUNCTIONS OP^ CROSS-STRIATED MUSCLES of different size, a large one (12 X G em.) applied to the breast, and a small one( cm. diameter) applied over the motor point to be tested. Suppose nowthe large electrode is the anode: the current enters then with relatively lowdensity, spreads out through the body with still less density and finally collectsat the cathode with great density. Since now the effects of a current dependupon its density, it follows that with currents of moderate strength these effectswill appear only at the smaller electrode. Some of the many threads of currentreaching 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 currentapplied to the skin over the nerve, after de Watteville. through the nerve under it. The effective cathode of the current lies where thesethreads pass out of the nerve, and if, as we have assumed, the smaller electrodeis the cathode, other things being eqmd, the current will have its greatest possi-ble density there. If the current is reveised so that it now enters the body by thesmaller electrode (which is still over the nerve), the places where the threads ofcurrent leave the nerve constitute as before the effective cathode; the densityof the current now however is less than in the first case (Fig. 165). The polar lav: of excitation applies also to muscle, both with the constantand 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 Eng
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