92 



EX l'KRIMENTAL PHYSIOLOGY 



Fig. 78 shows a nionochord fitted for convenience on a small base, and 

 consisting of a wire wound in a zigzag round pegs, its two ends being 



attached to the binding screws a and 

 b. The electrodes are connected 

 to a and the movable contact c. 

 In the battery circuit a murcury 

 commutator, k 1 , with cross wires 

 is interposed, so that the direction 

 of current can be readily reversed. 

 A key, k 2 , is also inserted, so that the 

 current can be closed and opened as 

 required. 



Another instrument that is fre- 

 quently used for varying the strength 

 of current is the rheochord. This 

 consists, in its simplest form, of two 

 wires, s 1 a and s 2 b (fig. 79), stretched 

 parallel to one another on a wooden 

 base. To the ends, s 1 and s 2 , binding 

 screws are fixed, and the wires are 

 connected by a metal slider, c, which 

 can be pushed along the wires. In 

 the figure it is represented connected to send a branch current to a pair of 

 unpolarisable electrodes across which a nerve lies. A battery current is sent 

 through the rheochord, a commutator, k 1} being interposed, and a key for 

 making and breaking the circuit at k,. The current on reaching s 1 divides, 

 passing through the rheochord to s 2 or through the nerve. The amount 



Fig. 78. — A Second Form of Monochord. 



Fig. 



79. — The Eheochord as Arranged for Varying the Direction 

 and Strength of a Current through a Nerve. 



passing through the nerve varies directly with the resistance of its deriving 

 circuit, the rheochord. By increasing this resistance more current is sent 

 through the nerve, and on diminishing it less. 



In more complex forms of the instrument the range of the rheochord is 

 increased by adding other wires by means of which more resistance can be 

 thrown into the rheochord circuit than can be reached by the single pair of 

 wires. 



Suppose, for the sake of example, that the resistance of the electrodes and 

 nerve is 100,000 ohms, and the resistance of the rheochord 5 ohms ; then 

 tctS&SSj of the total current passes through the rheochord, and rdhnr? through 

 the nerve. 



Experiment 11. — Fit up the apparatus as in fig. 79. Attach the muscle 

 to a myograph lever and arrange it to record twitches on a stationary drum. 



