136 PHYSIOLOGICAL CHEMISTRY 



substances, muscle contains various extractives and salts, such 

 as creatine, urea, inosite, taurine, and also some lipins. After 

 activity, particularly if the muscle supply of blood or oxygen 

 is low, lactic acid is found. This is believed to be a product 

 of the partial oxidation of glucose. The lactic acid may be fur- 

 ther oxidized, or rebuilt into something else. 



The important property of muscles is their power of contract- 

 ing. The process has been extensively studied, but as yet it is 

 but imperfectly understood. Heat is liberated, glucose is used 

 up and C0 2 is given off. Possibly the swelling and shortening 

 of the muscle fibers is caused by the acid and heat. The in- 

 dividual muscle fibers become shorter and thicker. The acid 

 is quickly destroyed, C0 2 is given off, and relaxation follows. 

 If the acid is not removed, cramps or rigor result. It has been 

 suggested that the swelling of the fibers is caused by taking up 

 water under the influence of acid, since proteins swell in acid 

 solution. The question is still unsettled, however. 



A great deal of interest has centered around the chemistry of 

 muscle activity, and the factors involved in the control of the 

 blood supply to the muscles. Studies of the respiratory ex- 

 change, the composition of the blood and the changes taking 

 place in excised muscles under various conditions all have con- 

 tributed to our information, but many of the problems connected 

 with muscle contraction still are unsolved. 



During activity the blood supply to the muscles is greatly 

 increased. If the activity is general, this is brought about by 

 increased heart action and by dilation of the blood vessels in the 

 muscles themselves. If the activity is local, the increase in 

 blood supply is brought about mainly in the second manner. 

 Even during rest a state of tonus exists, and C0 2 is produced 

 and 2 consumed. Indeed this is the case even if the nerve 

 supply is cut so that the muscle loses its tone. In this case the 

 extent of oxidation is greatly decreased. During contraction, 

 the amounts of C0 2 and 2 are very greatly increased. The 

 nerve centers are stimulated, though it is not known exactly 

 how. An increase in the free C0 2 in the blood, and <a slight 



