128 STUDIES IN ADVANCED PHYSIOLOGY. 



This movement up and down occupies about nine-hun- 

 dredths of a second, making the entire time about one-tenth 

 of a second. Just what these molecular changes in the 



Fig. 67. CURVE OF A SINGLE CONTRACTION. 



R, a, the latent period; a, 6, the gradual contraction to a maximum; b, c, the some- 

 what more sudden relaxation ; c, d, a slight rise owing to the inertia of the recording 

 lever, and having nothing to do with the muscle contraction itself. 



muscle are, which occur during the latent period, it is im- 

 possible to tell, but it is interesting to note that by means 

 of a galvanometer (an instrument for detecting currents of 

 electricity) electrical currents are indicated in the muscle 

 running from the parts furthest removed from the nerve to 

 the part directly stimulated by the nerve. These currents 

 are called the currents or waves of negative variation. What 

 meaning these currents have which flow in a muscle during 

 the latent period is not known, but they are external evi- 

 dences of complicated molecular changes in the muscle sub- 

 stances itself, which make possible the succeeding contrac- 

 tion. Such a contraction which follows a single stimulus 

 is called a simple contraction. Such a simple contraction 

 occurs, as long as the vitality of the muscle is left intact, 

 as often as a stimulus is applied. If, however, the stimuli 

 are applied so rapidly that the second stimulus enters the 

 muscle, before the relaxation from the first stimulus has 

 occurred, the muscle contracts still more. It is, so to speak, 

 a superposition of the second contraction upon the first. 

 Such a superposition does not, however, go on indefinitely; 

 that is, there cannot be super-imposed a third onto a second, 

 or a fourth onto a third indefinitely, for soon a maximum is 

 reached beyond which, no matter how many stimuli are sent 

 in, the muscle will not contract. 



