CHAP. IL] THE CONTRACTILE TISSUES. 149 



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 after this they are 

 thawed, they are at first supple and as we have seen may be made 

 to yield muscle plasma; but they very speedily enter into rigor 

 mortis of a most pronounced character. 



85. The influence of blood supply. When a muscla 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 clotting of the muscle substance, it 

 is easy to understand that the amount of rigidity, i.e. the amount 

 of the clot, and the rapidity of the onset, i.e. the quickness with 

 which clotting takes place, may vary independently. The rapidity 

 of onset after muscular exercise and wasting disease may perhaps 

 be, in part, dependent on an increase of acid reaction, which is 

 produced under those circumstances in the muscle, for this seems 

 to be favourable to the clotting 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 sub- 

 sequent supply of blood. Thus when 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 ; the return of the blood stream 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, though they have lost their irritability, have not yet 

 advanced into rigor mortis. At such a juncture a renewal of the 

 blood stream may restore the irritability of those fibres which 



