GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 107 



ilar differences are found to exist between different muscles in other 

 in the turtle, for instance, as is shown by the myograms in Fig. 38. 



It is interesting to connect the rate of the contraction process in different 

 muscles with their function. The omohyoid muscle of the turtle is capable of 

 comparatively rapid contractions, and the action of this muscle is to draw back 

 the head beneath the projecting shell; the pectoralis, on the other hand, 

 although strong, contracts slowly ; it is a muscle of locomotion and has to 

 move the heavy body of the animal. Unstriated muscles, which are remark- 

 able for the slowness and the duration of their contractions, are found chiefly 

 in the walls of the intestines, blood-vessels, etc., which require to remain in a 

 state of continued contraction for considerable periods and do not need to alter 

 rapidly. It is the business of the heart-muscle to drive fluids often against 

 considerable resistance, and a strong, not too rapid, slightly prolonged contrac- 

 tion, such as is peculiar to it, would be best adapted to its function. The bulk 

 of the muscles of the bodies of warm-blooded animals are capable of rapid 

 contraction and relaxation, but the rate normal to the muscle is found to vary 

 with the form of work to be done. The muscles which control the vocal 

 organs, for instance, have a very rapid rate of relaxation as well as of con- 

 traction. The muscles which move the bones appear to have different rates 

 of contraction and relaxation according to the weight of the parts to be moved ; 

 those which control the lighter parts, as the hand, being capable of rapid con- 

 tractions, while those which have to overcome the inertia of heavier parts, to 

 which rapidity of action would be a positive disadvantage, react more slowly. 

 In general, where rapid, brief, and vigorous contractions are required, pale 

 striated muscles are found; where more prolonged contractions are needed, 

 red striated muscles occur. The accompanying myograms (Fig. 39) illustrate 



100 



100 



FIG. 39.-A, maximal contractions of the gastrocnemius medialis of the rabbit (pale muscle), weighted 

 with 50, 100, 300, and 500 grams ; B, maximal contractions of the soleus of the rabbit (red muscle), we 

 with 50, 100, and 200 grams (after Cash). 



the difference in the rate of contractions of pale and red striated muscles of 

 the rabbit. 



Pale and red striated fibres are found united in the same muscle 

 instances, and in these cases it is supposed that the former, which are capal 



