VARIATIONS IN THE SINGLE CONTRACTION. 461 



striated slow-contracting muscles found in the hollow viscera 

 have a latent period of some seconds. The striated muscles of 

 cold-blooded animals have a longer latency than the same kind 

 of muscle in birds and mammalia. The same gastrocnemius of a 

 frog has a shorter latent period when strongly stimulated, or when 

 its temperature is raised, and vice versa. 



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. 



FIG. 186. 



Curves drawn by the same muscle in different stages of fatigue. A, when 

 fresh; B, C, D, E, each immediately after the muscle had contracted 200 

 times. Showing that fatigue causes a low, long contraction. 



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 extremes in 

 the rapidity of muscle contraction may be found in the contrac- 

 tile 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 " " 



Drag<n-fly, .... 28 " " 



Butterfly, .... 9 



