THE CHEMISTRY OF MUSCLE. 63 



myogen represent the protein constituents of the contractile ele- 

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

 sarcoplasm. The proteins 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. It is a synthetic 

 reaction in which the simple molecule of the monosaccharide 

 (dextrose) is converted by dehydration and condensation to the 

 larger molecule of the polysaccharide (glycogen). It is repre- 

 sented in principle by the reaction 



n(C 6 H 12 6 ) n(H 2 0) = (C 6 H 10 O 5 )n. 



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 

 is consumed by the tissue during its activity, and it is assumed 

 that before it is thus consumed it is converted back into sugar by 

 the action of an amylolytic enzyme contained in the muscle. The 

 glycogen, therefore, itself represents a local deposit of carbohydrate 

 nutritive material. The sugar and the glycogen must be con- 

 sidered as one from the standpoint of the nutrition of the muscle. 

 During muscular activity the store of glycogen is used up, and if 

 the activity is sufficiently prolonged it may be made to disap- 



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

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

 von Forth, "General Review, Handbuch der Biochemie," vol. 2, part 2, p. 244. 



