128 INFLUENCE OF ACTIVITY. [Book i. 



e. g. from 28 metres to 1 metre per sec. At about 0°, the irrita- 

 bility of the nerve disappears altogether. 



"When a muscle is exposed to similar cold, e. g. to a tempera- 

 ture very little above zero, the contractions are remarkably pro- 

 longed ; they are diminished in height at the same time, but not 

 in proportion to the increase of their duration. Exposed to a 

 temperature of zero or below, muscles soon lose their irritability, 

 without however undergoing rigor mortis. 



§ 80. 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 recognised 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. 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. Mere loss of irritability, even though 

 complete, if stopping short of the actual clotting of the muscle 

 substance, ma; be with care removed. 



The influence of blood supply cannot be so satisfactorily studied 

 in the case of nerves as in the case of muscles ; there can however 

 be little doubt that the effects are analogous. 



§ 81. The influence of functional activity. This too is more 

 easily studied in the case of muscles than of nerves. 



When a muscle within the body is unused, it wastes ; when 

 used, it (within certain limits) grows. Both these facts shew that 

 the nutrition of a muscle is favourably affected by its functional 

 activity. Part of this may be an indirect effect of the increased 

 blood supply which occurs when a muscle contracts. When a 



