PHYSIOLOGIC APPARATUS. 



675 



consists of a flattened glass tube attached to a universal joint and supported 

 by an insulated brass stand. The lower end of the tube is closed with 

 kaolin or China clay made into a paste with a 0.6 per cent, solution of 

 sodium chlorid. It can be molded into any desired shape. The interior 

 of the tube is partially filled with a saturated solution of sulphate of zinc 

 in which is immersed the strip of amalgamated zinc. To the upper end 

 of the zinc the conducting wire is attached. 



The v. Fleischl brush electrode is similar to the preceding except that 

 the end of the tube is closed by the brush of a camel's-hair pencil. 



The d' Arson val electrode consists of a glass tube containing a silver 

 rod coated with fused silver chlorid. The interior of the tube is filled 

 with normal salt solution 0.6 per cent, and 

 the end closed with a thread or plug of 

 asbestos which is made to project beyond 

 the tube for a short distance. 



Any one of the these three electrodes is 

 suitable for physiologic experimentation, as 

 their free ends neither corrode the tissues nor 

 develop electric currents. 



Keys. Muscle and nerve tissues are con- 

 ductors of electricity. When, therefore, the 

 terminals (the non-polarizable electrodes) of 

 the wires of a cell 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 necessary to keep the elec- 

 trodes in contact with the tissues for a varia- 

 ble 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. 338). 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 interposing this 

 key in the circuit. 



1. As a Simple Key. For this purpose one of the wires, usually the nega- 



tive, 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 



FIG. 338. Du BOIS-REY- 

 MOND FRICTION KEY. 



