206 



EXPERIMENTAL PHYSIOLOGY. 



Example. Suppose the length of nerve to be 4 cni., and the 

 time required for the impulse to travel from b to a to be 

 T 1 TT sec. Then we have 4 : 100 : ^ : ^", or 30 metres (about 

 90 feet) per second, as the velocity of nerve energy along a 

 nerve. 



2. Repeat the observation with the pendulum myograph. 

 Practically the same arrangements are necessary. 



If it be desired to test the effect of heat or cold on the rapidity 

 of propagation, the nerve must be laid on ebonite electrodes, 

 made in the form of a chamber, and covered with a lacquered 

 copper plate on which the nerve rests. Through the chamber 

 water at different temperatures can be passed, and the effect on 

 the rate of propagation observed. 



3. Unequal Excitability of a Nerve Apparatus. Battery, two 

 keys, wires, commutator, induction machine, two pairs of 

 electrodes. 



Fig. 94. Scheme for the Unequal Excitability of a Nerve. 



(a.) Arrange the apparatus as in Fig. 94, introducing a 

 Morse key in the primary circuit. Dissect out the whole 

 length of the sciatic nerve with the foot attached. Lay the 

 nerve on two pairs of electrodes, A and B, one near the 

 muscle, and the other away from it, and as far apart as 

 possible. Two pairs of wires thrust through a cork or piece 

 of caoutchouc will do quite well. 



(b.) Stimulate the nerve at A with a strength of current 

 that gives just a minimal contraction. Reverse the conimu- 



