60 



PHYSIOLOGY OF MUSCLE AND NERVE 



rated solution of sulphate of zinc into which is placed a short rod of amalga- 

 mated zinc carrying the end of the copper wire. At their points of contact with 

 the muscle or nerve a small tuft of cotton should be placed which has been 

 thoroughly moistened with saline solution. These electrodes must be carefully 

 washed after each experiment and must always be kept in saline solution for several 

 hours before they are used in order to render the clay completely permeable. 

 Polarization is impossible in this case, because at the junction of the cathodal metal- 

 lic zinc with the liquid conductor ZnSO4, the cation Zn deposits itself upon the zinc 

 electrode and does not act upon the water to liberate hydrogen gas. In quite the 

 same way, the anode is kept free, because there the sulphion SOi does not attack 

 the water but the zinc, forming ZnSC>4. 



The Making and Breaking of the Current. The electrodes are 

 always permitted to remain in contact with the muscle-nerve prep- 

 aration, while the making and breaking of the current is accomplished 



B 



FIG. 30. THE MAKING AND BREAKING OF THE CURRENT BY MEANS OF A DuBois- 



REYMOND KEY (K). 



by the closing and opening of a key or switch, interposed between the 

 positive pole of the battery and the positive electrode. The DuBois- 

 Reymond key consists of two bars of brass connected by a rocking 

 plate. If arranged as is shown in figure 30 A, the current is made to 

 pass through the muscle by closing this bridge, while its opening 

 breaks the circuit. If arranged as is represented in figure 30#, the 

 bridge remains down to begin with. The current then flows from the 

 anode to the cathode of the battery through the key and does not 

 reach the muscle at all, because the resistance offered by the tissue 

 between the points of contact of the electrodes, is very much greater 

 than that resident in the brass bridge. Conversely, if the key is 

 raised, the current must seek its level by way of the longer course 

 through the muscle, while its closure again permits the current to 

 seek the battery by following the path of least resistance through the 

 brass bridge. By the latter procedure the current is short-circuited. 



