158 CHANGES IN MUSCLE. [CH. xin. 



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 sarco- 

 lactic acid, and in part to the formation 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 the suggestion that in such cases at 

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

 It is known that pepsin (absorbed from the alimentary canal) is 

 present in muscle, and that this ferment will act in an acid medium. 

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

 fore present, as the reaction of rigorcd muscle is acid. 



Order of Occurrence. The muscles are not affected simul- 

 taneously by rigor mortis. It affects the neck and lower jaw 

 first ; next, the upper extremities, extending from above down- 

 wards ; and lastly, reaches the lower limbs ; in some rare 

 instances it affects the lower extremities before, or simul- 

 taneously 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. 



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 irri- 

 tability, induce an early occurrence of the rigidity, while con- 

 ditions by which the disappearance of the irritability is delayed, 

 are succeeded by a tardy onset of this rigidity. Hence its speedy 

 occurrence, and equally speedy departure in the bodies of persons 

 exhausted by chronic diseases ; and its tardy onset and long 

 continuance after sudden death from acute diseases. 



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. 



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. 



