PRACTICAL EXERCISES 625 



(Fig. 167, p. 560). Make a muscle-nerve preparation from a large 

 frog (preferably a bull-frog), so that the sciatic nerve may be as long 

 as possible. Connect the knock-over key with the primary circuit of 

 an induction machine, which should contain a single Daniell cell. 

 Arrange two pairs of fine electrodes under the nerve on the myograph 

 plate, one near the muscle, the other at the central end. Connect 

 the electrodes with a Pohl's commutator (without cross-wires), the 

 side-cups of which are joined to the terminals of the secondary coil, 

 as shown in Fig. 195 . By tilting the bridge of the commutator the 

 nerve may be stimulated at either point. Great care must be taken 

 to keep the nerve in a moist atmosphere by means of wet blotting- 

 paper ; but at the same time it must not lie in a pool of normal 

 saline, as twigs of the stimulating current would in this case spread 

 down the nerve, and we could never be sure that the apparent was 

 always the real point of stimulation. The writing-points of the lever 

 and tuning-fork having been adjusted to the smoked plate, as in 16, 

 the bridge of the Pohl's commutator is arranged for stimulation of 

 the distal point of the nerve, the plate is shot with the short-circuit- 



FIG. 195. ARRANGEMENT FOR MEASURING THE VELOCITY OF THE NERVE- 

 IMPULSE. 



A, travelling plate of spring myograph; M, muscle lying on a myograph plate; 

 N, nerve, lying on two pairs of electrodes, E and E' ; C, Pohl's commutator without 

 cross wires ; K, knock-over key of spring myograph (only the binding-screws shown) ; 

 K', simple key in primary circuit ; B, battery ; P, primary coil ; S, secondary coil. 



ing key in the secondary closed, and an abscissa line and time curve 

 traced. Then the writing-point of the fork is removed and the plate 

 again shot with the key in the secondary open, and a muscle- curve 

 is obtained. The commutator is now arranged for stimulation of 

 the central end of the nerve, and another muscle-curve taken. 

 Vertical lines are drawn through the points where the two curves 

 just begin to separate out from the abscissa line. The interval 

 between these lines corresponds to the time taken by the nerve- 

 impulse to travel along the nerve from the central to the distal pair 



40 



