522 HANDBOOK OF PHYSIOLOGY. 



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 oc- 

 currence 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 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 exist- 

 ence of paralysis in a part, provided the paralysis has not been attended 

 with very imperfect nutrition of the muscular tissue. 



The rigidity affects the involuntary as well as the voluntary muscles, 

 whether they be constructed of striped or unstriped fibres. The rigidity 

 of involuntary muscles with striped fibres is shown in the contraction of 

 the heart after death. The contraction of the muscles with unstriped 

 fibres is shown by an experiment of Valentin, who found that if a grad- 

 uated tube connected with a portion of intestine taken from a recently- 

 killed animal, be filled with water, and tied at the opposite end, the 

 water will in a few hours rise to a considerable height in the tube, 

 owing to the contraction of the intestinal walls. It is still better shown 

 in the arteries, of which all that have muscular coats contract after 

 death, and thus present the roundness and cord-like feel of the arteries 

 of a limb lately removed, or those of a body recently dead. Subsequently 

 they relax, as do all the other muscles, and feel lax and flabby, and lie 

 as if flattened, and with their walls nearly in contact. 



Action of the Voluntary Muscles. 



The greater part of the voluntary muscles of the body act as sources 

 of power for moving levers, the latter consisting of the various bones to 

 which the muscles are attached. 



Examples of the three orders of levers in the Human Body. All levers have 

 been divided into three kinds, according to the relative position of the power. 

 the weight to be removed, and the axis of notion or fulcrum. In a lever of 

 the first kind the power is at one extremity of the lever, the weight at the other, 



