586 A MANUAL OF PHYSIOLOGY 



good, or better^ method is to use pressure for the extraction 

 of the plasma from the frozen fragments of muscle. A low 

 temperature is essential, otherwise the plasma will coagulate 

 rapidly within the injured muscle. 



A similar plasma can be expressed from the skeletal 

 muscles of warm-blooded animals (Halliburton), and with 

 greater difficulty from the heart. Attempts to obtain it 

 from smooth muscle have hitherto failed, possibly because 

 of the unfavourable anatomical conditions. 



When the muscle, after exhaustion with water, is covered with a 

 solution of a neutral salt, a 5 per cent, solution of magnesiuno 

 sulphate or 10 per cent, solution of ammonium chloride being the 

 best, certain proteids are extracted which on dilution clot or are 

 precipitated much in the same way as the muscle-plasma obtained 

 by cold and pressure ; and the process is hastened by keeping them 

 at a temperature of 40 C. 



In the extracts of mammalian muscle before coagulation has 

 occurred three chief proteids are present : paramyosinogen, coagu- 

 lated by heat at 47 to 50 C. ; myosinogen, coagulating at 55 to 

 60 C. (usually about 56) ; and serum-albumin, coagulating about 

 73. In extracts of frog's muscle there is in addition a substance 

 which coagulates at about 40. Both the paramyosinogen and the 

 myosinogen, but particularly the former, show a tendency, even at 

 ordinary temperatures, to pass into an insoluble form myosin. At 

 body temperature this transformation occurs more quickly. The 

 myosin precipitate, which rapidly forms in muscle-plasma, is some- 

 times called the muscle-clot, and the liquid which is left the muscle- 

 serum. A similar myosin precipitate or clot is 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 this precipitate is either soluble in saline solutions or 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 such solutions. Some have 

 supposed that in the living muscle paramyosinogen and myosinogen 

 exist as such. But this is not certain. They may simply be formed 

 when the muscle dies. 



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

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

 which breaks up both at the death of the muscle and during con- 

 traction, and that, indeed, contraction is only a transient and remov- 

 able rigor (Hermann). But it cannot be admitted that there is any 

 fundamental connection between rigor and contraction, although 

 there are some superficial resemblances. In both there is (i) shorten- 

 ing; (2) heat-production; (3) formation of fixed acid and carbon 

 dioxide ; (4) electrical changes. Another analogy might be forced 

 into the list by anyone who was determined to see only rigor in 



