96 THE HUMAN BODY 



(ten centimeters per second). When the point is opposite t, stimu- 

 late the muscle by an electric shock; the result, until the muscle has 

 fully lengthened again, will be the curve tuvwxy, from which many 

 things may be learned. In the first place we see that the muscle 

 does not commence to contract at the very instant of stimulation, 

 but at an appreciably later time, and during the interval the lever 

 draws the horizontal line tu; this period, occupied by preparatory 

 changes within the muscle, is known as the latent period. Then the 

 muscle begins to shorten and the lever to rise, until the summit of 

 the contraction is reached at w. The muscle then, but only grad- 

 ually passes back to the resting state, tracing the line wxy. The 

 curve shows three distinct phases in the contraction: the latent 

 period; the period of shortening; the period of relaxation. Know- 

 ing the rate of horizontal movement, we can measure off the time 

 occupied by each phase. The horizontal distance from t to u repre- 

 sents the time taken by the latent period; from u to z, the time 

 occupied in shortening; from z to y, the time taken in elongation; 

 in a fresh frog's muscle these times are respectively T ^, T ^ 7 , T Jo 

 of a second. In the muscles of warm-blooded animals they are all 

 shorter, but the difficulties in the way of accurate experiment are 

 very great. If we know the relative lengths of the arms of the 

 lever we can of course readily calculate from the height, wz, of the 

 curve the extent of shortening of the muscle. With a single elec- 

 trical stimulation this is never more than one-fourth the total 

 length of the muscle. 



In Fig. 47 the accessory apparatus used in practice to indicate 

 on the moving surface the exact instant of stimulation and to 

 measure the rate at which S moves have been omitted. 



The Influence on Contraction Height of Increasing Stimula- 

 tion Strength. If an isolated muscle is stimulated at regular 

 intervals, as once in two seconds, by induction shocks made 

 stronger each time, there will be at first, if the shocks to begin 

 with are weak enough, no visible response. Presently, as stronger 

 shocks are used, barely perceptible twitches are given. These 

 become higher and higher as the stimuli are increased until a point 

 is reached beyond which no further increase in height appears, no 

 matter how much the intensity of the shock may be increased. 

 These highest contractions which the muscle is capable of giving 

 as the result of any single stimulus, however strong, are called 



