PRACTICAL EXERCISES 



713 



17. Velocity of the Nerve-impulse. Use the spring myograph 

 (Fig. 228, p. 643) or a very rapidly rotating drum. 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 com- 

 mutator (without cross-wires), the side-cups of which are joined to 

 the terminals of the secondary coil, as shown in Fig. 265. 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 or a moist 

 chamber ; but at the same time it must not lie in a pool of salt 

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

 down the nerve, and 



we could never be 

 sure that the appar- 

 ent was always the 

 real point of stimu- 

 lation. The writing- 

 points of the lever 

 and tuning-fork hav- 

 ing been adjusted to 

 the smoked plate, as 

 in 12 (p. 710), the 

 bridge of the Pohl's 

 commutator is ar- 

 ranged for stimula- 

 tion of the distal 

 point of the nerve, 

 the plate is shot 

 with the short-cir- 

 cuiting 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 

 of electrodes. Its value in time is given by the tracing of the tuning- 

 fork. The length of the nerve between the two pairs of electrodes 

 is now carefully measured with a scale divided in millimetres, and the 

 velocity calculated (p. 689). 



18. Chemistry of Muscle. Mince up some muscle from the hind- 

 legs of a dog or rabbit (used in some of the other experiments), of 

 which the bloodvessels have been washed out by injecting 0*9 per cent, 

 salt solution through a cannula tied into the abdominal aorta until 

 the washings are no longer tinged with blood. To some of the 

 minced muscle add twenty times its bulk of distilled water, to another 



FIG. 



264. RHYTHMICAL CONTRACTIONS OF 

 PHAGUS OF CHICKEN (BOTAZZI). 



CEso- 



