54 



HUMAN PHYSIOLOGY 



To find the duration of the contraction, a tuning fork, 

 vibrating 100 times per second, may be made to record its 

 vibrations on the surface. (Practical Physiology, Chap. 



in.) 



In this way such a tracing as is shown in Fig. 22 is pro- 

 duced. 



From this it is evident that the muscle] does not contract 

 the very moment it is stimulated, but that a short latent 

 period supervenes between the stimulation and the contrac- 



tion. In the muscle of the 

 frog attached to a lever this 

 usually occupies about T^th 

 second ; but if the change in 

 the muscle is directly photo- 

 graphed without any lever 



FIG. 22. Trace of Simple Muscle 

 Twitch (1) showing periods of 

 latency, contraction, and relaxa- 

 tion ; record of moment of stimu- 

 lation (2) ; and a time record made 

 with a tuning fork vibrating 100 

 times per second (3). 



being attached to it, this 

 period is found to be very 

 much shorter. 



The latent period is fol- 

 lowed by the period of con- 

 traction. At first it is sudden, 

 but it becomes slower, and finally stops. Its average dura- 

 tion in the frog's muscle is about -j-J^-th second. 



The period of relaxation follows that of contraction, and 

 it depends essentially on the elasticity of the muscle, whereby 

 it tends to recover its shape when the distorting force is re- 

 moved. The recovery is therefore at first fast and then slow, 

 and it lasts in the frog's muscle about To 4rth second. 



The whole contraction thus lasts only about T Vth second 

 in the frog's muscle. In mammalian muscle it is much 

 shorter, and in the muscle of insects shorter still. 

 2nd. Extent of Contraction. 



While, as will be afterwards considered, the extent of con- 

 traction is modified by the strength of stimulus and the state 

 of the muscle, the total amount of contraction is primarily 

 determined by the length of the muscle. If a muscle of two 

 inches contracts to one-half its length, the amount of con- 

 traction is one inch, but if a muscle of four inches contracts 

 to the same amount, it shortens by two inches. 



3rd. Force of Contraction is measured by finding what 



