MUSCLE 673 



soluble modification (soluble myosin), which coagulates at 40 C., 

 and seems to be identical with the protein coagulating at that 

 temperature which can be extracted from frog's muscles. At body- 

 temperature the transformation occurs more quickly. The myosin 

 precipitate, which rapidly forms in muscle-plasma, is sometimes 

 called the muscle-clot, and the liquid which is left the muscle-serum. 

 A similar myosin precipitate or clot seems to be formed in the 

 interior of the muscular fibres in natural rigor and in the rapid 

 rigor produced by heating a muscle to a little above the body- 

 temperature. But in natural rigor the whole of the paramyosinogen 

 and myosinogen do not undergo the change, since a certain amount 

 of these substances can as a rule be extracted from dead muscle by 

 saline solutions. Thus, in rabbit's muscles, before the onset of rigor 

 mortis, 87-3 per cent, of the total protein was found to be soluble 

 in 10 per cent, ammonium chloride solution, and only 12' 7 per cent, 

 coagulated ; while after rigor had occurred, 71*5 per cent, was 

 coagulated, and only 28-5 per cent, remained soluble (Saxl) . It is not 

 known whether in the living muscle paramyosinogen and myosinogen 

 exist as such. Certain facts suggest that muscle contains only one 

 protein (Mellanby) . It has, indeed, been stated that if a tracing is 

 taken from a muscle which is gradually heated, it first shortens at 

 the temperature of coagulation of paramyosinogen, and then again 

 at that of myosinogen, and that in frog's muscle there is an addi- 

 tional shortening at 40. the temperature of coagulation of the 

 soluble myosin. The conclusion has been drawn that these sub- 

 stances must be present as such in the living fibres, and that the 

 successive shortenings are mechanical phenomena due to their heat 

 coagulation. Similar shortenings have been described in nerve 

 and liver tissue at about the temperatures at which the proteins 

 in extracts of these tissues are coagulated by heat. But Meigs has 

 shown that the supposed correspondence is far from being exact, 

 and^that muscles whose proteins have been already coagulated in 

 a mixture of alcohol and salt solution still show the typical shortening 

 on being heated. The heat shortening is, therefore, dependent on some 

 other process than aggregation of the particles of coagulable protein. 



It has been suggested that myosin (or its precursors), lactic 

 acid, and carbon dioxide are all products of some complex body 

 which breaks up both during contraction and at or before the 

 death of the muscle, and that, indeed, contraction is only a 

 transient and removable rigor (Hermann). But it cannot be 

 admitted that there is any fundamental connection between 

 rigor and contraction, although there are some superficial resem- 

 blances. In both there is (i) shortening ; (2) heat-production ; 

 (3) formation of lactic acid and carbon dioxide ; (4) electrical 

 changes. Another analogy might be forced into the list by any- 

 one who was determined to see only rigor in contraction : the 

 rigor passes off as the contraction passes off, although the ' reso- 

 lution ' of a rigid muscle takes days, the relaxation of an active 

 muscle a fraction of a second. The disappearance of Jrigor is not 

 dependent on putrefaction ; it takes place when growth of bacteria 

 is prevented^ (Hermann). Possibly it is connected with autolytic 

 processes jlue to intracellular ferments (p. 509). 



43 



