266 THE BODY AT WOKK 



traction can be based upon an analogy between muscle and 

 any known mechanical contrivances for generating power, not 

 even excluding apparatus designed for the purpose of measuring 

 osmotic force. 



If living muscle is frozen, pounded with snow containing 

 0*6 per cent, of sodic chloride, and placed upon a filter, a fluid 

 plasma passes through the filter as the mixture thaws. Like 

 blood-plasm, it clots spontaneously without, however, so far 

 as is known the intervention of a ferment. 



All muscles become rigid after death, owing to the coagula- 

 tion of their plasma. It used to be thought that contraction 

 was a stage towards rigidity a stage from which muscle, so 

 long as it is alive, recovers. This view was based upon the 

 fact that exhausted muscle such, for example, as that of a hare 

 which has been coursed becomes rigid much sooner than 

 rested muscle. But this phenomenon has a different explana- 

 tion. The setting of muscle in rigor mortis is due to the de- 

 velopment of lactic acid (one of the waste-products of active 

 muscle). The more there is of this ready formed at the time 

 of death, the more quickly does coagulation of muscle-plasma 

 occur. The formation of lactic acid is due to deficiency of 

 oxygen. So long as muscle obtains as much oxygen as it 

 wants, its metabolism is complete. The oxidized products 

 which it loses are water and carbonic acid. This is true also 

 of the changes which occur after death. If a strip of muscle 

 is hung in an atmosphere of oxygen, it forms no lactic acid, 

 and it does not become rigid. If, on the other hand, the supply 

 of oxygen has run short before death occurred, rigor mortis sets 

 in very quickly. A dead frog takes a long while in becoming 

 rigid, and its rigidity is transient. Until the moment of 

 death the frog is taking up oxygen through its lungs, and even 

 after death it probably takes it, as it does when it is alive, 

 through the skin. A fish becomes rigid very quickly. For some 

 time after it is caught it continues to live ; but, being unable 

 to breathe in air, every molecule of oxygen which was in its 

 body when it left the water is used up before it dies. 



In the human body rigor mortis usually sets in from two 

 to four hours after death, and lasts about two days ; but both 

 the rapidity of its appearance and its duration depend upon 

 various circumstances. As muscles become rigid they con- 



