748 THE PHYSIOLOGY OF THE CONTRACTILE TISSUES 



Calcium salts have the opposite effect (Loeb). Sodium (and in a minor 

 degree lithium) salts have a peculiar relation to the contraction of 

 skeletal muscle, for which they appear to be indispensable. Yet sodium 

 chloride produces a paralyzing action on the frog's motor nerve-endings, 

 so that after perfusion with a solution of that salt stimulation of the 

 motor nerve causes no contraction, or with a slighter degree of paralysis 

 contraction only after a long interval. The effect can be counteracted 

 by solutions containing calcium salts (Locke, Gushing). 



Rigor Mortis. When a muscle is dying, its excitability, after 

 perhaps a temporary rise at the beginning, diminishes 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 com- 

 pletely 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 

 contraction 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 sometimes 

 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 ammonium 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. Fiirth'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. The 



