420 HANDBOOK OF PHYSIOLOGY. 



of the trunk, is produced a general stiffening of the body, constituiug 

 the rigor mortis or post-mortem rigidity. 



When this condition has set in, the muscle (a) becomes acid in reaction 

 (due to development of sarco-lactic acid), (b) gives off carbonic acid in 

 great excess, (c) Its volume is slightly diminished; (d) the muscul&Y fibres 

 become shortened and opaque, and their substance sets firm. It comes 

 on much more rapidly after muscular activity, and is hastened by 

 warmth. It may be brought on, in muscles exposed for experiment, by 

 the action of distilled water and many acids, also by freezing and thaw- 

 ing again. 



Cause. The immediate cause of rigor seems to be a chemical one,, 

 namely, the coagulation of the muscle plasma. We may distinguish, 

 three main stages (1.) Gradual coagulation. (2.) Contraction of coagu- 

 lated muscle-clot (myosin), and squeezing out of muscle-serum. (3.) 

 Putrefaction. After the first stage, restoration is possible through the 

 circulation of arterial blood through the muscles, and even when the 

 second stage has set in, vitality may be restored by dissolving the coagti- 

 lum of the muscle in salt solution, and passing arterial blood through its 

 vessels. In the third stage recovery is impossible. 



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 only, it affects the lower extremities 

 before, or simultaneously with, the upper extremities. It usually ceases 

 in the order in which it began: first at the head, then in the upper ex- 

 tremities, and lastly, in the lower extremities. It never 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 fibre into the condi- 

 tion 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 occur- 

 rence of the rigidity, while conditions 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 experi- 

 ments of Brown-Sequard make it probable that the rigidity may 



