CHAP, ii.] THE CONTRACTILE TISSUES. 147 



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 

 conies 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 myosin, it is easy to under- 

 stand that the amount of rigidity, i.e. the amount of the clot, 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 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 coagulation 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, 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 were not yet rigid, aud thus appear to do 

 away with rigor mortis ; yet it appears that in such cases the 

 fibres which have actually become rigid never regain their irrita- 

 bility, but undergo degeneration. 



Mere loss of irritability, even though complete, if stopping short 

 of the actual coagulation of the muscle substance, may be with 

 care removed. Thus if a stream of blood be sent artificially 

 through the vessels of a separated (mammalian) muscle, the irrita- 

 bility may be maintained for a very considerable time. On stopping 

 the artificial circulation, the irritability diminishes and in time 

 entirely disappears ; if however the stream be at once resumed, 

 the irritability will be recovered. By regulating the flow, the 



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