474 



MUSCLE-NERVE PHYSIOLOGY 



The electrical current is made continuous by the use of the two fluids in the 

 following manner. The action of the dilute sulphuric acid upon the zinc plate 

 partly dissolves it and liberates hydrogen, and this gas passes through the 

 porous vessel and decomposes the copper sulphate into copper and sulphuric 

 acid. The former is deposited upon the copper plate, and the latter passes 

 through the porous vessel to renew the sulphuric acid which is being used up. 

 The copper-sulphate solution is renewed by crystals of the salt, which are 

 kept on a little shelf attached to the copper plate and slightly below the 

 level of the solution in the vessel. The current of electricity supplied by this 

 cell will continue without variation for a considerable time. Other cells, 

 such as the dry cell (which, however, is adapted to open-circuit work) may be 

 used in place of Daniell's. The way in which the apparatus is arranged is to 

 attach wires to the copper and zinc plates and to bring them to a key connect- 



FIG. 315. Du Bois Reymond's Key. 



FIG. ^16. Mercurv Kev. 



ing the wires of the battery. One often employed' is Du Bois Reymond's, 

 figure 315. It consists of two pieces of brass about an inch long, in each of 

 which are two holes for wires and binding-screws to hold them tightly. These 

 pieces of brass are fixed upon a vulcanite plate to the under surface of which 

 is attached a screw clamp by which it can be secured to the table. The 

 interval between the pieces of brass can be bridged over by means of a third 

 thinner piece of similar metal fixed by a screw to one of the brass pieces, and 

 capable of movement by a handle at right angles so as to touch the other piece 

 of brass. If the wires from the battery are brought to the inner binding- 

 screws, and the bridge connects them, the current passes across it and back 

 to the battery. Wires are connected with the outer binding-screws, and the 

 other ends are joined together for about two inches, but, being covered 

 except at their points, are insulated ; the uncovered points are about an eighth 

 of an inch apart. These wires are the electrodes, and the electrical stimulus is 

 applied to the muscle through them, if they are placed behind its nerve. 

 When the connection between the two brass plates of the key is broken by 

 depressing the handle of the bridge, the key is then said to be opened. 



