38 HAND-BOOK OF PHYSIOLOGY. 



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

 (due to sarco-lactic acid), and gives off carbonic acid in great excess. Its. 

 volume is slightly diminished: the muscular fibres become shortened 

 and opaque, and their substance has set 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 thawing again. 



Cause. The immediate cause of rigor seems coagulation of the 

 muscle plasma (Briicke, Kuhne, Norris). We may distinguish three 

 main stages. (1.) Gradual coagulation. (2.) Contraction of coagulated 

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

 tion. 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 coagulum 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 post-mortem contraction. 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 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 extremities, and lastly in the lower extremities. It never com- 

 mences earlier than ten minutes, and never later than seven hours, after 

 death; and its duration is greater in proportion to the lateness of its ac- 

 cession. Heat is developed during the passage of a muscular fibre 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 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 super- 

 vene immediately after death, and then pass away with such rapidity as- 

 to be scarcely observable. 



Experiments. Brown-Sequard took five rabbits, and killed them by 



