162 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



as compared with normal resting muscle. To this may be added that, as has 

 been said, the muscle continues to be irritable even when rigor is quite 

 advanced, and that it enters into rigor more quickly if left in connection with 

 the central nervous system. 



On the other hand, one cannot fail to be impressed with the differences 

 between the two forms of contraction. 



Normal Contracting Muscle. 

 Contains uncoagulated proteids. 

 Is translucent. 



Is soft and flexible. 



Is no less elastic than in repose. 



Is more extensible than in repose. 



Contracts rapidly. 



Fatigues rapidly and relaxes. 



Muscle contracting by Rigor Mortis. 

 Contains coagulated myosin. 

 Is opaque. 

 Is firm and still! 

 Is less elastic than before. 

 Is less extensible than before. 

 Contracts very slowly, as a rule. 

 Remains contracted a long time. 



Furthermore, it may be added that normal contractions only occur when 

 the irritable muscle is stimulated, while a muscle can enter into rigor when its 

 irritability has been taken away by subjecting it to oxalate solutions, 1 also, 

 when it has been curarized and so shut out from all nervous influences. 2 



Rigor is not confined to the voluntary muscles, though it is less easily 

 observed in the case of most involuntary muscles. The heart enters rapidly 

 into rigor, with the formation of sarcolactic acid. The non-striated muscle 

 of the stomach and ureters, too, has been seen to undergo this change. 



The passing off of rigor mortis is usually accompanied by beginning 

 decomposition, and, indeed, it has been thought that the decomposition is 

 the cause of softening of the muscle. This is denied by certain observers, 

 and it is stated that rigor may pass off when the presence of putrefactive 

 organisms is excluded by special aseptic precautions. Another explanation 

 is that a process of intramuscular digestion goes on. Pepsin, a proteolytic 

 ferment, lias been found in small amounts in the muscle; and acid, which is 

 necessary to the action of this digestive ferment, is formed in the muscle 

 when it coagulates. The presence of these substances would make the diges- 

 tion of proteid material possible, and the fact that proteoses and peptone, 

 products of such digestion, are to be found in the muscle after death, though 

 not present during life, favors the view. It cannot be considered, however, 

 to be definitely established. 



The rigidity of muscles in rigor can be readily broken up by stretching 

 and massaging the muscles, and when this has been done it does not return. 

 The Chemical Changes which accompany the Development of Rigor. — Rigor 

 mortis is characterized by the coagulation of a part of the muscle-substance; 

 this can be prevented by a temperature a little below 0° C. Cold, although 

 temporarily depriving the muscle of its irritability, does not, unless extreme and 

 long-continued, kill the muscle protoplasm. Frogs can be frozen stiff and 

 recover their activity when they thaw out. Indeed, this probably happens not 



1 Howell : Journal of Physiology, 1893, vol. xiv. p. 476. 



2 Nagel : l'jlihjrr s Archir, Bd. Iviii. S. 279. 



