136 THERMAL AND CHEMICAL CHANGES IN MUSCLE [CH. XII. 



Rigor Mortis. 



After death, the muscles gradually lose their irritability and pass 

 into a contracted condition. This affects all the muscles of the body, 

 and usually fixes it in the natural posture of equilibrium or rest. 

 The general stiffening thus produced constitutes rigor mortis or post- 

 mortem rigidity. 



The cause of rigor is the coagulation of the muscle-plasma, which 

 is more fully described in the next section. This coagulation results 

 in the formation of myosin, and is gradual in onset. Simultaneously 

 the muscles (a) become shortened and opaque, (b) heat is evolved, (c) 

 they give off carbonic acid, and (d) become acid in reaction ; this is due 

 in part to the formation of sarcolactic acid, and in part to the forma- 

 tion of acid phosphates. 



After a varying interval, the rigor passes off, and the muscles are 

 once more relaxed. This sometimes occurs too quickly to be caused 

 by putrefaction, and there is very little doubt that it is really the 

 first stage in the self-digestion or autolysis which occurs in all tissues 

 after death, owing to the presence of intracellular enzymes or fer- 

 ments. It is known that a pepsin-like or proteolytic enzyme is 

 present in muscle, as in many other animal tissues, kidney, spleen, 

 etc. (Hedin), and that such enzymes act best in an acid medium. 

 The conditions for the solution of the coagulated myosin are there- 

 fore 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 com- 

 mences 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. 



