GENERAL MUSCLE PHYSIOLOGY 



189 



called the latent period. The contraction occurs at first 

 with increasing and then with decreasing rapidity till the 

 maximum is reached ; after this the muscle relaxes, at first 

 rapidly, but soon more slowly until it has acquired its former 

 length. Frequently the relaxation is not complete, especially 

 when the load of the muscle is small (see Fig. 9). 



The length of the latent period for the skeletal frog- 

 muscle is, at room temperature, about o.oi second, for 

 human muscle 0.004 t o.oi second, for smooth muscle 0.4 

 to 0.8 second. 



Z 



FIG. 9. ISOTONIC CONTRACTION OF A FROG-MUSCLE. 



J", curve of contraction ; r, moment of stimulation; from r to a, latent period; 

 from a' to b, period of increasing energy; from b to <:, period of decreasing 

 energy; Z, time-curve produced by a vibrating tuning-fork (each vibration 

 equals o.oi second). 



The duration of contraction for a skeletal muscle of a frog 

 at room temperature is about o. I to o. 1 5 second, for a human 

 muscle a little less, for a smooth muscle I to 3 minutes. 

 Various striated muscles of the same animal contract with 

 different rapidity, e.g. the gastrocnemius of a frog contracts 

 more rapidly than the hyoglossus. Many animals (rabbits, 

 birds) have slowly contracting striated muscles which have 

 a red color and are poor in sarcoplasm, while they also have 

 rapidly contracting muscles which are white and contain 

 much sarcoplasm. 



The extent of the contraction (height of contraction) in a 

 maximal contraction of a frog-muscle is about one-fifth of 

 the length of the fibre. 



Conditions influencing the contraction. 



I. Temperature. Between the temperature of 40 and 

 -f- 40 C. the duration of the contraction and the latent 



