GENERAL PHYSIOLOGY OF MUSCLE AND NERVE. 109 



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 parte, 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. 42) illustrate 



mo 



Fig. 42. — 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), weighted 

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



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

 the rabbit. Ranvier says the latent period of red muscle of rabbit is four 

 times as long as that of the pale ; and Tigerstedt states the latent period of 

 red muscles of the frog to be 0.02 second and of the pale muscles 0.005 

 second. 



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

 instances, and in these eases it is supposed that the former, which are capable 

 of very rapid and powerful but short-lived contractions, start the movement, 

 while the slower red muscles continue it. Bottazzi ' would explain many of 

 the peculiarities of muscle contraction on the theory that both the isotropic 

 and anisotropic substances are contractile, and that they react differently 

 under varving conditions. The isotropic substance, the Barcoplasma, is 

 responsible for the slow, prolonged movements of the muscle and the aniso- 

 tropic substance for the rapid, brief movements. In ordinary contractions 

 they both act, though to different degrees. 



(6) Effect of Tension Caused by Weights and Myograph-lever on the Extent 

 and Course of the Contraction. — As we have seen, the rate of the contraction 

 of an ordinary striated muscle is much too rapid to be followed by the eye. 

 and to study the course of the change in form it is necessary to employ sonic 

 kind of recording mechanism. Every mechanical device for recording the 

 movements of the muscle has inertia, and, if given motion, acquires momen- 

 1 Bottazzi : Journalof Physiology, 1897, xxi. p, 1. 



