94 VARIATIONS OF IRRITABILITY. [BOOK i. 



without however undergoing rigor mortis. After an exposure of 

 not more than a few seconds to a temperature not much below 

 zero, they may be restored, by gradual warmth, to an irritable con- 

 dition, even though they may appear to have been frozen. When 

 kept frozen however for some few minutes, or when exposed for a 

 less time to temperatures of several degrees below zero, their 

 irritability is permanently destroyed. When thawed, they enter 

 into rigor mortis of a most pronounced character. 



The Influence of Blood-Supply. 



When a muscle still within the body is deprived by any means 

 of its proper blood-supply, as when the blood-vessels going to it are 

 ligatured, the same gradual loss of irritability and final appearance 

 of rigor mortis are observed as in muscles removed from the body. 

 Thus if the abdominal aorta be ligatured, the muscles of the lower 

 limbs lose their irritability and finally become rigid. So also in 

 systemic death, when the blood-supply to the muscles is cut off by 

 the cessation of the circulation, loss of irritability ensues, and rigor 

 mortis eventually follows. In a human corpse the muscles of the 

 body enter into rigor mortis in a fixed order : first those of the jaw 

 and neck, then those of the trunk, next those of the arms, and 

 lastly those of the legs. The rapidity with which rigor mortis 

 comes on after death varies considerably, being determined both by 

 external circumstances and by the internal conditions of the body. 

 Thus external warmth hastens and cold retards the onset. After 

 great muscular exertion, as in hunted animals, and when death 

 closes wasting diseases, rigor mortis in most cases comes on rapidly. 

 As a general rule it may be said that the later it is in making its 

 appearance, the more pronounced it is, and the longer it lasts ; but 

 there are many exceptions, and when the state is recognized as 

 being fundamentally due to a coagulation, it is easy to understand 

 that the amount of rigidity, i.e. the amount of the coagulum, and 

 the rapidity of the onset, i.e. the quickness with which coagulation 

 takes place, may vary independently. The rapidity of onset after 

 muscular exercise and wasting disease is apparently dependent on 

 an increase of acid reaction, being produced under those circum- 

 stances in the muscle, for this seems to be favourable to the coagu- 

 lation of the muscle plasma. When rigor mortis has once become 

 thoroughly established in a muscle through deprivation of blood, it 

 cannot be removed by any subsequent supply of blood. Thus 

 where the abdominal aorta has remained ligatured until the lower 

 limbs have become completely rigid, untying the ligature will not 

 restore the muscles to an irritable condition ; it simply hastens the 

 decomposition of the dead tissues by supplying them with oxygen 

 and, in the case of the mammal, with warmth also. A muscle 

 however may acquire as a whole a certain amount of rigidity on 

 account of some of the fibres becoming rigid, while the remainder, 



