RIGOR MORTIS. 473 



are removed, the more work a muscle can do without becoming 

 exhausted. 



2. Temperature has a marked effect on the irritability as well 

 as form of contraction of muscles. Very low temperatures 

 approaching zero C. diminish the irritability of a muscle, but 

 do not seem to tend toward more rapid exhaustion. High tem- 

 peratures approaching 30 C. increase the irritability, and at 

 the same time rapidly bring about fatigue. At about 35 C. an 

 isolated frog's muscle begins to pass into heat tetanus, and soon 

 loses its irritability forever. 



3. Functional activity is accompanied by an increased blood 

 supply, and a more perfect nutrition of the muscles, and hence 

 use is advantageous for their growth and power ; while, on the 

 other hand, continued and prolonged inactivity causes a lowering 

 of the nutrition and loss of irritability. Thus, when the nerves 

 supplying the voluntary muscles are injured, there is considerable 

 danger of atrophy and tissue degeneration of the muscles ; the 

 contractile substance becomes replaced by fat granules. This 

 degeneration also occurs in the stump when a limb is amputated, 

 the distal attachments of the muscles having been cut, they atro- 

 phy ; for, although their nervous supply is uninjured, they cannot 

 act, and after some time muscle tissue can hardly be recognized 

 in them. 



DEATH RIGOR. 



The death of muscle tissue is preceded by, and associated with, 

 a set of changes which are a kind of exaggeration of those 

 observed in its active state. The most obvious phenomenon is 

 an unyielding contraction, which causes the stiffening of the 

 body after systemic death. Hence it is called rigor mortis. The 

 muscles harden, lose their elasticity, and the tissue is torn if 

 forcibly stretched. When isolated, the muscle is seen to be 

 opaque, and its reaction is found to be distinctly acid. A con- 

 siderable quantity of heat is developed during the progress of 

 the rigor. The electric currents alter in direction, and finally 

 disappear. 



The period at which rigor comes on, as well as the time it lasts, 

 depend on (a) the state of the muscles themselves, and (6) the 

 40 



