474 . MANUAL OF PHYSIOLOGY. 



circumstances under which they are placed at the time of death. 

 All influences which tend to facilitate the approach of tissue death 

 also tend to induce early and rapidly-terminating rigor, viz. : (1) 

 Prolonged activity as may be shown in a muscle artificially 

 tetanized, or may be seen in an animal whose death was preceded 

 by intense muscular exertion causes rigor to appear almost im- 

 mediately, and .to terminate rapidly. (2) Within certain limits, 

 a high temperature facilitates the production of rigor in dying 

 muscles, and, indeed, a temperature not much exceeding that 

 normal to the tissue induces rigor immediately. This form of 

 contraction, which is called heat rigor, is brought about in mam- 

 malian muscles by a temperature of about 50 C., and in frogs' 

 muscles below 40 C. If, however, the temperature .of a muscle 

 be suddenly raised to the boiling point, it is killed, and the chief 

 phenomena of rigor are prevented from occurring. (3) Freezing 

 postpones the appearance of the changes in the muscles upon 

 which rigor depends. (4) Stretching, or any mechanical exci- 

 tation which tends to injure or hasten the death of the tissue, 

 causes it to pass more rapidly into rigor. (5) The application 

 of water and of a number of chemical substances causes muscles 

 quickly to pass into a state of rigor similar in all essential respects 

 to that which ordinarily follows the death of the tissue. (6) Any 

 stoppage in the blood current normally flowing through a muscle, 

 after some little time makes it pass into a state of rigidity like 

 rigor mortis, but this may be removed by allowing the blood to 

 flow freely again through the muscle. 



It is generally admitted that muscle rigor depends on the 

 coagulation of the muscle plasma, giving rise to myosin and 

 muscle serum. This is in most respects comparable with the 

 coagulation of the blood, and also seems to be produced by the 

 action of some ferment, of which several have been made out in 

 dead muscle tissue (compare the par. on chemistry, p. 445). 



Most of the phenomena of the process of muscle rigor remind 

 us of the changes which were noted as occurring in muscle when 

 it passes from the passive to the active state. Thus the short- 

 ening of the fibres, the evolution of heat, and the chemical 

 changes may be said to be identical in contraction and rigor 



