250 



PRACTICAL PHYSIOLOGY. 



[L 



In usiiig the instrument, connect a Daniell's cell to the binding screws at 

 A and B, and to the same screws attach the wires of the electrodes over 



which the nerve (c d) of the muscle 

 (F) is laid. We have two circuits 

 {a c d h and a A B fe) ; the wires of 

 the rheochord are introduced into the 

 latter. 



Push up the slider with its cups (L) 

 until it touches the two brass plates 

 I and 2, and insert all the plugs 

 (8,-85) in their places, thus making 

 the several blocks of brass practically 

 one block. In this position, the zero 

 of the instrument, the resistance offered 

 by the rheochord circuit is so small as 

 compared with that including the nerve, 

 that practically all the electricity passes 

 through the former and none through 

 the latter. 



Move the slider away from A, when 

 a resistance is thrown into the rheo- 

 chord circuit, according to the length 

 of the platinum wires thus introduced 

 into it, and so a certain fraction of the 

 current is sent through the electrode 

 circuit. If the plug 8, be taken out, 

 more resistance is introduced, that due 

 to the German-silver wire (I //), and, 

 therefore, a certain amount of the 

 current is made to pass through the 

 electrode circuit. By taking out plug 

 after plug more and more resistance is 

 thrown into the rheochord circuit. The 

 plugs are numbered, and the diameter 

 and length of the German-silver wires 

 are so selected in making the instru- 

 ment, that the resistances represented 

 plugs when removed are all multiples of the resistance 

 the slider moves. Proceed taking out plug 

 The result, and explanation thereof, are 



Fig. 



175. — R.heochord of Du 

 Bois-Reymond. 



by the several 



in the platinum wires on which 

 after plug, and note the result. 

 referred to in Lesson XLIX. 1. 



LESSON L. 



VELOCITY OF NERVE-IMPULSE IN FROG, MAN- 

 DOUBLE CONDUCTION IN NERVE— KUHNE'S 

 GRACILIS EXPERIMENT, &c. 



1. Velocity of Nerve-Energy in a Frog's Motor Nerve. 



The rate of propagation of a nerve-impulse or excitatory change 

 may be estimated by either the pendulum or spring-myograph. 



