464 MANUAL OF PHYSIOLOGY. 



The latent period is considerably lengthened by fatigue. If 

 the weight be so applied that it does not extend the muscle before 

 contraction, but only bears on it the instant it commences to 

 shorten, the duration of the latent period increases in proportion 

 to the weight the muscle has to lift. 



The duration of the single contraction of striated muscle varies 

 in different cases and under varying circumstances. The greatest 

 difference is reached by the muscles found in different kinds of 

 animals. The contraction of some kinds of muscle tissue (non- 

 striated muscle of mollusca, for example) occupies several 

 minutes, and reminds one of the slow movement of protoplasm ; 

 while the rapid action of the muscle of the wing of a horsefly 

 occurs 330 times a second. Various gradations between these 



FIG. 187. 



Six curves drawn by the same muscle when stretched by different weights. Showing 

 that as the weight is increased the latency becomes longer and the contraction less in 

 height and duration. 



extremes in the rapidity of muscle contraction may be found in 

 the contractile tissues of different animals. The following table 

 gives the rate of contraction of some insects' muscles, which may 

 help to show the extent of these variations. 



Horsefly, 330 contractions per second. 



Bee 190 " " 



Wasp, 110 



Dragon-fly, 28 " " 



Butterfly, 9 



Among the vertebrata the duration of the contraction of the 

 skeletal muscles varies considerably according to the habits of the 

 animal. The limb muscles of the tortoise and toad take a very 



