42 PRACTICAL PHYSIOLOGY 



in experiments for recording a single maximal contraction on a rapidly 

 revolving drum (p. 24); it is only necessary in addition to place a 

 second ' striker ' in the primary circuit through the drum. If the rate 

 of revolution of the drum remains constant, then, by simply altering 

 the angular distance between the two ' strikers/ a second stimulus can 

 be sent in at varying intervals after the first. Make a gastrocnemius 

 preparation and stimulate it either directly or through its nerve. Set 

 the drum in motion and, with the Du Bois key open, approximate the 

 1 strikers' until the muscle clearly to the eye just responds with a 

 complete contraction to each stimulus. Close the Du Bois key, bring the 

 writing point on to the bottom of the drum, describe a base line and 

 mark on it the point at which each stimulus enters the preparation ; 

 then open the key, record both contractions, and close the key again. 

 Now raise the myograph until the writing point will just clear the top 

 of the curves, approximate the strikers a little, and again record the 

 contractions, after marking a base line and the points of entrance of 

 the two stimuli. This process is repeated until the * strikers ' are finally 

 at such a distance apart that the second stimulus falls within the latent 

 period of the first. 



In this way Figs. 46 A and B were obtained. It shows that when a 

 second maximal stimulus reaches a muscle during any part of its period 

 of relaxation or of shortening, the rest of the contraction due to the first 

 stimulus is omitted and the muscle starts off on a fresh contraction in 

 response to the new stimulus. Since the second contraction may be as 

 high as the first and starts with the writing point above the base line, 

 it follows that the height of the second twitch above the abscissa is 

 greater than and may be nearly double that of a single contraction ; 

 in other words, a summation of contraction has taken place. If, how- 

 ever, the second stimulus falls within the latent period of the first, then 

 the muscle responds by a contraction only to the first stimulus 

 (Fig. 46 A) ; that is, the muscle is refractory to the second stimulus 

 so far as its being able to respond by a second contraction is concerned ; 

 therefore in skeletal muscle the ' refractory ' period corresponds in time 

 to the latent period (cf. the 'refractory' period of cardiac muscle, 

 p. 64). 



Genesis of Tetanus. In order to study the response of a muscle to a 

 series of stimuli, it is necessary to have an apparatus which will auto- 

 matically make and break the primary circuit of an induction coil at any 

 desired rate. 



The vibrating reed is a convenient form and consists of a flat steel 

 spring which can be clamped in various positions along its length ; by 

 -altering the length of spring which is allowed to vibrate, the number of 



