THE CHEMISTRY OF MUSCLE. 61 



as they are known to us seem to be obviously incomplete. Ac- 

 cording to von Fiirth, two proteins may be obtained from mam- 

 malian muscle by extracting it with dilute saline solutions, namely, 

 myosin and myogen, the latter existing to three or four times the 

 amount of the former. Myosin belongs to the globulin group of 

 proteins (see appendix) ; it is coagulated by heat at 44 to 50 C., 

 it is precipitated by dialysis or by weak acids, it is easily precipi- 

 tated from its solutions by adding an excess of neutral salts, such 

 as sodium chlorid, magnesium or ammonium sulphate. With 

 the last salt it is completely precipitated when the salt is added 

 to one-half saturation or less.' Its most interesting property, how- 

 ever, is that on standing at ordinary temperatures it passes over 

 into an insoluble modification which separates out as a sort of 

 clot. Following the terminology used for the blood, this insoluble 

 modification is called myosin fibrin. Myogen, the other protein, 

 seems to fall into the group of albumins rather than globulins. 

 It is not precipitated by dialysis and requires more than half 

 saturation with ammonium sulphate for its complete precipitation. 

 It is coagulated by heat at a temperature of 55 to 65 C. Solutions 

 of myogen on standing also undergo a species of clotting, the in- 

 soluble protein that is formed in this case being called myogen fibrin. 

 It appears, however, that in changing to myogen fibrin the myogen 

 passes through an intermediate stage, designated as soluble myogen 

 fibrin, in which its temperature of heat coagulation is as low as 

 30 to 40 C., the lowest temperature recorded for any protein. 

 As was stated in the paragraph on muscle rigor, it is known that 

 frog's muscle goes into heat rigor at about 37 to 40 C., and in 

 accordance with this fact it is stated that a protein, soluble my- 

 ogen fibrin, which is not present in mammalian muscle, occurs 

 normally in the muscle of the frog and also of the fishes. 



It may be doubted whether these proteins exist as such in 

 the living muscle. Extracts must of necessity be made after the 

 muscle plasma is dead and probably coagulated. Myogen is said 

 not to occur in the 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 con- 

 cerned, 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 solu- 

 tions 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 



