PRACTICAL EXERCISES 79! 



of tetanus, as in Experiment 14, p. 789; (d) the relations between 

 strength of stimulus and amount of contraction. For this last experi- 

 ment the drum should be stationary while the contraction is being 

 recorded, and should be allowed to move a little between successive 

 contractions. Begin with the secondary at such a distance from the 

 primary that a contraction is just caused by a break shock. Then 

 gradually increase the strength of the stimulus (always using the break) 

 till maximum contraction is obtained. The gradual increase in the 

 response is very clearly seen with the cesophageal preparation (Waller). 



For further experiments on the contraction of smooth muscle, see 

 pp. 66 and 447. 



1 6. Velocity of the Nerve-Impulse. Use the spring myograph 

 (Fig. 244, p. 720) 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 



Fig. 282. Arrangement for Measuring the Velocity of the Nerve -Impulse. A, travel- 

 ling 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. 



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 (with- 

 out cross-wires), the side-cups of which are joined to the terminals of 

 the secondary coil, as shown in Fig. 282. 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 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 n (p. 788), 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-circuiting 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 



