The death of muscle tissue is associated with a set of changes which, in some respects, resemble those observed in its active state. The most obvious phenomenon is an unyielding contraction, which causes the stiffening of the body after death. Hence, it is called rigor mortis. The muscles harden; lose their elasticity, and the tissue is torn if forcibly stretched. When isolated, the muscle is seen to be opaque, and its reaction is found to be 40 distinctly acid. A considerable quantity of heat is developed during the progress of the rigor. The electric currents alter in direction and finally disappear.

The period at which rigor comes on and its duration depend on (a) the state of the muscles, and (b) the circumstances under which they are placed at the time of death. All influences which tend to cause death of the tissue induce early rigor of short duration, viz., (i) Prolonged activity - as may be shown in a muscle artificially tetanized, or seen in an animal whose death was preceded by intense muscular exertion - causes rigor to appear almost immediately, and to terminate rapidly. (2) High temperature facilitates the production of rigor in dying muscles; indeed, a temperature not much exceeding that normal to the tissue induces rigor. This form of contraction, which is called heat rigor, is brought about in mammalian muscles by a temperature of about 500 C, and in frog's muscles below 400 C. If, however, the temperature of a muscle be suddenly raised to the boiling point, it is killed, and the chief phenomena of rigor are prevented from occurring. (3) Freezing postpones the changes in the muscles upon which rigor depends. (4) Stretching, or any mechanical excitation which tends to injure the tissue, causes it to pass more rapidly into rigor. (5) The application of water and of a number of chemical substances cause muscles quickly to pass into a state of rigor similar to that which ordinarily follows the death of the tissue. (6) Any stoppage in the blood current normally flowing through a muscle, after some time makes it pass into a state of rigidity like rigor mortis, but this may be removed by allowing the blood to flow freely again through the muscle.

It is generally admitted that rigor mortis depends on the tendency of the muscle plasma to coagulate and give rise to myosin and muscle serum. This is, in most respects, comparable with the coagulation of the blood, and may also depend upon the action of some ferment, of which there is no lack in dead muscle tissue. Compare the paragraph on chemistry, pp. 445, 446.

Most of the phenomena of the process of muscle rigor remind us of the changes already described as occurring in muscle, when it passes from the passive to the active state. Thus, the shortening of the fibres, the evolution of heat, and the chemical changes may be said to be identical in contraction and rigor mortis. The electrical-changes are, however, very transitory, and the rigor is accompanied by loss of elasticity and irritability. Opacity of the tissue marks its later stages.

Thus, while dying, the muscle tissue may be said to go through a series of events analogous to those which would occur in a prolonged contraction without any period of recuperation. The idea has naturally suggested itself to the minds of physiologists, that the active state of muscle depends upon chemical changes which are the initial steps in the coagulation of the contractile substance, when the muscle is dying. The muscle tissue is supposed to contain a special proteid of extremely intricate and unstable chemical constitution, which is constantly undergoing slow molecular change, and which, if not reintegrated by constant assimilation, would pass into coagulation. Under the influence of stimuli a comparatively sudden and intense molecular disturbance is brought about, which produces shortening of the fibres, and the same chemical changes as precede the coagulation..Before the stage of coagulation appears a chemical rearrangement takes place, the result of which is the reconstruction of the unstable complex proteid. If nutriment be withheld, or if the stimulation be too powerful, the recovery cannot take place, and we find the muscle passing from a state of physiological contraction to one of intense exhaustion, and then to coagulation and death.