462 MUSCLE-NERVE PHYSIOLOGY 



It has been noticed that the relaxation in muscles after rigor sometimes 

 occurs too quickly to be caused by putrefaction. The suggestion that in 

 such cases the relaxation is due to a ferment-action is very plausible. 

 It is known that pepsin is present in muscles, and that this ferment will 

 act in an acid medium. The conditions for the solution of the coagulated 

 myosin are therefore present since the reaction of muscle in rigor is acid. 

 Subjecting fresh muscle to the action of heat (50 to 60 C.) or immersing 

 it in distilled water causes a similar coagulation to that of rigor mortis. 

 The former is known as heat rigor, and the latter as witer rigor. 



FIG. 332. Curve of Shortening of the Gastrocnemius Muscle of the Frog, During Heat Rigor. 

 The numbers indicate degrees centigrade. 



The muscles are not affected simultaneously by rigor mortis. It affects 

 the neck and lower jaw first; next, the upper extremities, extending from 

 above downward; and, lastly, reaches the lower limbs. In some rare instances 

 cnly, it affects the lower extremities before or simultaneously with the upper 

 extremities. It usually ceases in the order in which it begins: first at the 

 head, then in the upper extremities, and lastly in the lower extremities. It 

 never ordinarily commences earlier than ten minutes, and never later than 

 seven hours after death; and its duration is greater in proportion to the 

 lateness of its accession. Heat is developed during the passage of a muscular 

 fiber into the condition of rigor mortis. 



Since rigidity does not ensue until muscles have lost the capacity of being 

 excited by external stimuli, it follows that all circumstances which cause 

 a speedy exhaustion of muscular irritability induce an early occurrence of the 

 rigidity, while conditions by which the disappearance of the irritability is 

 delayed are succeeded by a tardy onset of the rigidity of rigor. This is the 

 explanation of its speedy occurrence, and equally speedy departure, in the 

 bodies of persons exhausted by chronic diseases; and its tardy onset and long 



