MUSCLES AND PHENOMENA OF CONTRACTION. 127 



the stimuli are applied to the nerve, and through this the 

 muscle induced to contract. If now such a muscle makes a 

 contraction, the same occurs with such rapidity that to the 

 unaided eye it seems but a rapid shrinking and a return to 

 its former position. If, however, the lower end of the 

 muscle be fastened to a lever, and this lever be made to 

 write on a drum revolving at a known speed, the contrac- 

 tion will pull the muscle and consequently the lever up, and 

 a curve will be described on the revolving surface. On this 

 curve the various phenomena are then at leisure determin- 

 able. The arrangement for such a demonstration may be 

 seen in the accompanying diagram. 



Fig. 66. ARRANGEMENT FOR SECURING MYOGRAMS. 



P, the moving recording surface, which by the projection at d opens the circuit in K 

 at c, and so induces a break current iap which passing through the muscle stimulates it. 

 In this way the moment of stimulation is determined exactly, it being the moment of 

 contact of d with c. The curve is then recorded as in figure. 



If now the exact moment be noted at which the impulse 

 reaches the muscle it is found that the muscle does not con- 

 tract at once, but that there is a short period after the im- 

 pulse reaches the muscle before it begins to shorten. This 

 period is about one-hundredth of a second, and is called the 

 latent period. During this period the nervous impulse has 

 probably originated molecular changes in the muscle pre- 

 paratory to the contraction. The latent period is succeeded 

 by a steady contraction of the muscle to a maximum, fol- 

 lowed by the relaxation of the muscle to its original position. 



