672 A MANUAL OF PHYSIOLOGY 



more and more until it ultimately responds to no stimulus, 

 however strong. The loss of excitability is not in itself a sure 

 mark of death, for, as we have seen, an inexcitable muscle may 

 be partially or completely restored ; but it is followed, or, 

 where the death of the muscle takes place very rapidly, perhaps 

 accompanied, by a more decisive event, the appearance of 

 rigor. The muscle, which was before soft and at the same time 

 elastic to the touch, becomes firm ; but its elasticity is gone. 

 The fibres are no longer translucent, but opaque and turbid. 

 If shortening of the muscle has not been opposed, it may be 

 somewhat contracted, although the absolute force of this con- 

 traction is small compared with that of a living muscle, and a 

 slight resistance is enough to prevent it. The reaction is now 

 distinctly acid to litmus. This is rigor mortis, the death- 

 stiffening of muscle. 



An insight into the real meaning of this singular and some- 

 times sudden change was first given by the experiments of 

 Kiihne. He took living frog's muscle, freed from blood, froze 

 it, and minced it in the frozen state. The pieces were then 

 rubbed up in a mortar with snow containing i per cent, of common 

 salt, and a thick neutral or alkaline liquid, the muscle-plasma, 

 was obtained by filtration. This clotted into a jelly when the 

 temperature was allowed to rise, but at o C. remained fluid. 

 The clotting was accompanied by a change of reaction, the 

 liquid becoming acid. An equally 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. 



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

 solution of a neutral salt, a 5 per cent, solution of magnesium sulphate 

 or 10 per cent, solution of ammonuim chloride being the best, certain 

 proteins are extracted which 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 three chief proteins are 

 present : paramyosinogen (v. Furth's myosin), coagulating by heat at 

 47 to 50 C. ; myosinogen (v. Furth's myogen), coagulating at 55 

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

 about 73. There is reason to believe that the serum-albumin 

 belongs to the blood and lymph, and is not a constituent of the 

 muscle-fibre. 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 (v. Furth's muscle fibrin). But whereas paramyosinogen 

 passes directly into myosin, myosinogen is first changed into a 



