676 TEXT-BOOK OF PHYSIOLOGY. 



the cell are carried to the inner holes of each block and then continued 

 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 cur- 

 rent 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. 339). 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 mova- 

 ble bent rod which may be 

 made to dip into or be with- 

 drawn 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. 33Q.-A MERCURY KEY. The Rheocord. With this 



apparatus an electric current 



may be divided, one portion continuing through a conductor back to the 

 battery, the other portion being sent off through the nerve. The strengths 

 of these two currents are inversely proportional to the resistances of their 

 circuits. A simple form of rheocord (Fig. 340) consists of a long wire 

 arranged for convenience in parallel lines on a small wooden base and 

 connected at its two ends with binding posts A and B. The resistance 

 of this wire, 1.6 ohms, can be increased by the introduction of small re- 

 sistance coils, between D and B, varying from 5 to 20 ohms. 



The two binding posts A and B are connected with the positive and 

 negative poles of an electric cell respectively. A simple key is placed in 

 the circuit. 



From A, a wire passes to one of the electrodes on which the muscle or 

 nerve rests. A second wire passes from the second electrode to a clamp 

 S, by way of the binding post C, which can be fastened to the long wire 

 at any given point. The current, on reaching A, will divide into two 

 branches, one of which will pass along the wire A, B, and thence back to 

 the cell; the other will pass through the nerve and back to S and thence- 



