MUSCLE 223 



being more spread out the greater the velocity of the 

 movement of the surface. 



(a) From such a trace 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 contraction. In the muscle of the frog, attached to a 

 lever, this usually occupies about 001 second ; but, if the 

 change in the muscle be directly photographed without any 

 lever being attached to it, this period is found to be very 

 much shorter, only about 0025 second. 



(b) The latent period is followed by the period of contrac- 

 tion. At first it is sudden, but it becomes slower, and 

 finally stops. Its average duration in the frog's muscle is 

 about 04 second. 



(c) 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 removed (p. 210). The recovery is therefore at first 

 fast and then slow, and it lasts in the frog's muscle about 

 5 second. 



The whole contraction thus lasts only about 0"1 second 

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

 shorter, in the rabbit about 0"07 second ; in the muscle 

 of insects shorter still, about 003 second. 



2nd. Extent of Contraction. 



From the height of the trace the actual shortening of the 

 muscle may be calculated by measuring the two limbs of the 

 lever (Practical Physiology). 



While, as will be afterwards considered, the extent of 

 contraction is modified by the strength of stimulus and the 

 state of the muscle, the total extent 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 (fig. 124, p. 243). 



Srd. Force of Contraction. 



The force of contraction, that is the tension developed, is 



