igB HUMAN PHYSIOLOGY 



Excised muscles of warm-blooded animals lose their 

 irritability in a few hours ; those of cold-blooded animals, at 

 a moderate temperature, in two to three days, while in a 

 lower temperature they retain their irritability for a long 

 time (as long as twelve days). Stoppage of circulation or 

 lack of oxygen soon destroys the irritability of muscles of 

 warm-blooded animals. 



Irritability is maintained only by the proper alternate 

 succession of rest and activity. On the one hand, the 

 irritability is lost by complete rest (e.g. in limbs which 

 remain at rest for a long time in fixed bandages); on the 

 other hand, the irritability is decreased by too great stimu- 

 lation. Section of the motor nerve after some time also 

 destroys the irritability of the muscles and causes it to 

 degenerate. 



Fatigue manifests itself by decrease in irritability and 

 ability to do work ; subjectively it manifests itself by painful 

 sensations in the muscle. The fatigue is due to: 



(1) Decomposition products (e.g. sarcolactic acid) pro- 

 duced by the prolonged activity of the muscle which 

 decrease the irritability. 



(2) The disappearance of material for furnishing energy. 

 If the fatigued muscle is allowed to rest, it recovers and 



the irritability increases by the removal of the fatigue-sub- 

 stance and by a fresh supply of oxidizable material. 



Rigor mortis. During the death of a muscle phenomena 

 similar to those of a contracting muscle appear, namely, 

 contraction (produced by the tension of the muscle in rigor 

 mortis), production of heat, consumption of oxygen, forma- 

 tion of carbon dioxide and lactic acid, disappearance of 

 glycogen, electrical phenomena. Rigor mortis has there- 

 fore been regarded as the last contraction of the dying 

 muscle. 



Besides the above-named processes, the coagulation of 

 myosinogen also takes place ; this causes the dead muscle 

 to have a whitish, cloudy appearance. 



The nervous system influences rigor mortis. Rigor mortis 



