THE CHEMISTRY OF MUSCLE. 59 



muscles of the invertebrates. It should be added that after the 

 most complete extraction with saline solutions the muscle fiber still 

 retains much protein material, and its structural appearance, so far 

 as cross-striation is concerned, remains unaltered. The portion of 

 protein material thus left in the muscle fiber as a sort of skeleton 

 framework is designated as the muscle stroma; it is not soluble 

 in solutions of neutral salts, but dissolves readily in solutions of 

 dilute alkalies. In striped muscle this so-called stroma forms 

 about 9 per cent, of the weight of the muscle; while in the heart 

 muscle it makes about 56 per cent., and in the smooth muscle, 

 72 per cent. It is at present uncertain whether the myosin and 

 myogen represent the protein constituents of the contractile ele- 

 ments of the muscle fibers or of the undifferentiated portion, the 

 sarcoplasm. The protein of plain muscle tissue and of cardiac 

 muscle have not received so much attention as those of voluntary 

 muscle. It is stated, however, that the proteins extracted from 

 these tissues by salt solutions are coagulable on standing, as in 

 the case of the extracts of voluntary muscle. In plain muscle 

 two proteins, in addition to some nucleoprotein, are described, 

 one belonging to the albumin and one to the globulin class, but 

 the identity or relationship of these proteins to those above de- 

 scribed has not been established. In heart muscle, myosin and 

 myogen occur in practically the same proportions as in voluntary 

 muscle, but the amount of stroma left undissolved after treatment 

 with saline solutions is, as stated above, much greater than in 

 skeletal muscle.* 



The Carbohydrates of Muscle. Muscle contains a certain 

 amount of sugar, dextrose or dextrose and isomaltose, and also 

 under normal conditions a considerable quantity of glycogen, or 

 so-called animal starch. The formation and the consumption of 

 glycogen in the body constitute one of the most interesting chapters 

 in the physiology of nutrition, and the relations of glycogen will 

 be treated more fully under that head. It may be stated here, 

 however, that the muscular tissue has the power of converting the 

 sugar brought to it by the blood into glycogen. This glycogenetic 

 action of the muscle is represented in principle by the reaction 



C 6 H 12 8 H 2 = C ? H 10 5 . 



Dextrose. Glycogen. 



The glycogen thus formed is stored in the muscle and forms 

 a constant constituent of well-nourished muscle in the resting 

 condition, the amount varying between 0.5 and 0.9 per cent, of 

 the weight of the muscle. The glycogen thus stored in the muscle 



* Vincent and Lewis, " Journal of Physiology," 26, 445, 1901 ; also " Zeit- 

 schrift f. physiolog. Chemie," 34, 417, 1901-2 ; Stewart and Sollman, loc. rit.; 

 Saxl, " Hofmeister's Beitriige," 1906, ix., 1. 



