PHYSIOLOGIC APPARATUS 



761 



are placed in contact with either a muscle or a nerve a circuit is made through 

 which a current of electricity flows; when one or both are removed, the 

 circuit is broken and the current ceases. In practical work it is often neces- 

 sary to keep the electrodes in contact with the tissues for a variable length 

 of time. The circuit, however, may be alternately made and broken at will 

 by interposing along the return wire a mechanic contrivance known as a key, 

 of which there are many forms. 



The du Bois-Reymond Friction Key. This consists of a plate of 

 vulcanite attached to a screw clamp by which it can be fastened to the edge 

 of a table (Fig. 336). The surface of the vulcanite 

 plate carries two rectangular blocks of brass, each 

 of which has two holes drilled through it, for the 

 insertion of wires, which are held in position by 

 small screws. A movable bridge of brass, provided 

 with an ebonite handle, serves to make connection 

 between the blocks. There are two ways of in- 

 terposing this key in the circuit. 



1. 4s a Simple Key. For this purpose one of the 



wires, usually the negative, is carried from the 

 cell to one block and then continued from the 

 second block. When the bridge is down, the 

 circuit is made and the current passes; when 

 it is up, the circuit is broken. 



2. As a Short-circuiting Key. When used for this 



purpose, the wires of the cell are carried to 

 the inner holes of each block and then con- 

 tinued from the outer holes to the tissues or 

 to some form of apparatus which it is desired 

 to actuate. When the key is closed, i.e., when 

 the bridge is down, the current on reaching 

 the key, will divide, one portion passing across 

 the bridge and so back to the cell, the other 

 portion passing to the tissue or apparatus. 

 The amount of the current which is returned 

 to the cell through the short circuit will be proportional to the resistance 

 of the longer circuit. As the latter is usually great in comparison with 

 the former, practically all the current is short-circuited. When the 

 bridge is lowered, therefore, the current is short-circuited; when it is 

 raised, the current flows into the longer circuit through the tissue or 

 apparatus. 



The Mercury Key. In this form the connection is established by means 

 of mercury. It consists of a circular block in the center of which there is a 

 cup containing mercury (Fig. 337). At opposite points there are binding 

 posts, one of which is provided with a rigid fixed copper rod passing into 

 the mercury; the other is provided with a movable bent rod which may 

 be made to dip into or be withdrawn from the mercury by the ebonite 

 handle. 



The effect of a constant or galvanic current on a muscle or nerve will 

 depend to some extent on its strength. This may be accurately regulated 

 by means of an apparatus known as 



FIG. 336. Du BOIS-REY- 

 MOND FRICTION KEY. 



