154 THERMAL AND CHEMICAL CHANGES IN MUSCLE [CH. XIII. 



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

 known that a pepsin-like or proteolytic ferment is present in muscle, 

 as in many other animal tissues, kidney, spleen, etc. (Hedin), and 

 that such ferments act best in an acid medium. The conditions for 

 the solution of the coagulated myosin are therefore present, as the 

 reaction of rigored muscle is acid. 



Order of Occurrence. The muscles are not affected simultaneously 

 by rigor mortis. It affects the neck and lower jaw first ; next, the 

 upper extremities, extending from above downwards; and lastly, 

 reaches the lower limbs ; in some rare instances 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 seldom commences earlier than ten minutes, or later than seven 

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

 lateness of its accession. 



The occurrence of rigor mortis is not prevented by the previous 

 existence of paralysis in a part, provided the paralysis has not been 

 attended with very imperfect nutrition of the muscular tissue. 



In some cases of sudden death from lightning, violent injuries, or paroxysms of 

 passion, rigor mortis has been said not to occur at all ; but this is not always the 

 case. It may, indeed, be doubted whether there is really a complete absence of 

 the post-mortem rigidity in any such cases ; for the experiments of Brown-Sequard 

 make it probable that the rigidity may supervene immediately after death, and 

 then pass away with such rapidity as to be scarcely observable. 



Chemical Composition of Muscle. 



The phenomena of rigor mortis will be more intelligible if we 

 consider the chemical composition of muscle. 



The connective tissue of muscle resembles connective tissue else- 

 where; the gelatin and fat obtained in analyses of muscle are 

 derived from this tissue. The sarcolemma is composed of a substance 

 which resembles elastin in its solubilities. 



The contractile substance within the muscular fibres is, during 

 life, of semi-liquid consistency, and contains a large percentage of 

 proteids and smaller quantities of extractives and inorganic salts. 

 By the use of a press this substance can be squeezed out of perfectly 

 fresh muscles, and it is then called the muscle-plasma. 



After death, muscle-plasma, like blood-plasma, coagulates (thus 

 causing the stiffening known as rigor mortis). The solid clot corre- 

 sponding to the fibrin from blood-plasma is called myosin, and the 

 liquid residue is called the muscle-serum. 



Pursuing the analogy further, it is found that the coagulation of 

 both muscle-plasma and blood-plasma can be prevented by cold, by 

 strong solutions of neutral salts, and by potassium oxalate, which 



