PRACTICAL EXERCISES 



817 



the spring fastened, so that only f , , , J of its length is free to vibraie. 



The rate of interruption of the primary circuit increases in proportion 



to the shortening of 

 the spring, and the 

 tetanus becomes more 

 and more complete, 

 till ultimately the 

 writing -point marks 

 an unbroken straight 

 line. Put on a time- 

 tracing by means of 

 an electro - magnetic 

 marker connected 

 with a metronome 

 beating seconds or 

 half-seconds (Fig. 88, 

 p. 195). (For speci- 

 men curves, see Fig. 

 267, p. 757-) 



15. Contraction of 

 Smooth Muscles 

 (i) Spontaneous 

 Rhythmical Contrac- 

 tions. Immerse in 

 oxygenated Ringer's 

 solution a ring of 



Fig. 289. Arrangement for Tetanus. A, upright with 

 notches, in which the spring S is fastened (shown in 

 section); C, horizontal board to which A is attached, 

 and in a groove in which the mercury-cup E slides. 

 The primary coil P is connected with E, and through 

 a simple key, K, with the battery B, the other pole 

 of which is connected with the end of the spring. 

 The wires from the secondary coil, P', go to a short- 

 circuiting key, K', from which the wires F go off to 

 the electrodes. 



oesophagus obtained 

 immediately after death from a cat, or, still better, from a chicken. Or 

 a segment of rabbit's intestine may be employed as described on p. 456. 

 Use the arrangement 

 described on p. 456. In 

 the case of the cat's oeso- 

 phagus the ring should 

 be taken from the lower 

 half of the oesophagus, 

 since the upper portion 

 contains purely striated 

 muscle. Obtain tracings 

 of the rhythmical con- 

 tractions on a slowly- 

 moving drum (Fig. 290). 

 (2) Fix one end of 

 a piece of cat's oeso- 

 phagus, 2 to 5 centi- 

 metres long, to a muscle- 

 clamp in a moist 

 chamber, and the other 

 end to a lever writing 

 on a drum. Connect 

 thin copper wires from 

 the secondary coil of an 



inductorium with the Fig. 290. Rhythmical Contractions of (Esophagus of 

 two ends of the piece Chicken (Botazzl). 



of oesophagus. Take 



tracings to show (a) the curve of a single contraction caused by a single 

 make or break shock, with estimation of the latent period, as in Experi- 

 ment ii, p. 815; (b) summation, as in Experiment 12, p. 816; (c) genesis 



52 



