STIMULATION OF MUSCLES AND OF NERVES 



419 



ing 1 in the nerve, muscle, etc., are desired, the polarization, which after a time 

 would be produced by the extraneous current, would greatly vitiate results. 



The discovery by Jules Regnauld that zinc in a concentrated solution of 

 zinc sulphate gives no polarization was of very great service in the development 

 of the methods of general nerve-muscle physiology. The fluid, however, must 



FIG. 157. Nonpolarizable electrodes, after Porter. Each electrode consists of a porous clay 

 " boot," which may be filled with saturated solution of Zn SO*. Connection with the battery 

 is made to the zinc bars placed inside the boot. A hollow place on the surface of the " toe " 

 is filled with normal saline and the nerve is laid across those two reservoirs in such a way 

 as to keep it continually moistened with the saline. The entire nerve-muscle preparation 

 can be kept moist by covering boots and all with a glass top which fits in the groove around 

 the edge of the porcelain base. 



not come in contact with the animal tissues, for they are completely destroyed 

 by so concentrated a solution. The current therefore is applied to the tissues 

 through porous-clay points molded into a suitable shape, and soaked with 0.6-per- 

 cent solution sodium chloride (Fig. 157). Such a mass is but slightly polariz- 

 able. Often the clay tip is sealed into the end of a glass tube filled with zinc- 

 sulphate solution into which amalgamated zinc bars connected with the source 

 of electricity are dipped. The boot-shaped electrodes represented in Fig. 157 

 themselves serve at once as the clay tip and the containers for the zinc sulphate. 

 When it is desired to localize the stimulus, or the connection with the galvanom- 

 eters very sharply, the tissue is connected with the porous-clay tips of the non- 

 polarizable electrodes by means of woolen threads wet with 0.6 per cent NaCl. 



It is presumed that the student is already acquainted with the principles of 

 the induction coil. If not, a text-book of physics should be consulted. 



Since the strength of the induction current depends on the abruptness with 

 which the primary current is changed, it is very important that closing and 

 opening the circuit should take place with equal precision. Many different kinds 

 of keys have been devised to supply this requirement; one is shown in Fig. 160. 



