54 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



lation of lactic acid, that it is, so to speak, a last tonic contraction, 

 the acid bringing; into action the mechanism of shortening as it does 

 in a normal contraction (p. 73). In addition to the rigor mortis 

 that occurs after death at ordinary temperatures, a condition of 

 rigor may be induced rapidly by raising the temperature of the 

 muscle to a certain point. Rigor induced in this way is designated 

 as heat rigor or rigor caloris. Much uncertainty has prevailed as to 

 whether heat rigor is different essentially from death rigor. Ac- 

 cording to some physiologists, the processes may be regarded as the ' 

 same, the heat rigor being simply a death rigor that is rapidly devel- 

 oped by the high temperature, this latter condition accelerating 

 the chemical changes leading to rigor, as is the case, for instance, 

 in the action of chloroform. This view is supported by a study 

 of the chemical changes that take place under the two condi- 

 tions, as will be described later, and by the fact that some of 

 the conditions that influence one phenomenon have a parallel 

 effect upon the other. For instance, death rigor is acceler- 

 ated by previous use of the muscle, and the same is true for heat 

 rigor. While a resting frog's muscle begins to go into heat rigor, 

 as judged by the shortening, at 37 to 40 C.; a muscle that has 

 been greatly fatigued shows the same phenomenon at 25 to 

 27 C.* According to other observers, heat rigor is due to an 

 ordinary heat coagulation of the proteins present in the muscle 

 fiber, and it has been claimed that a separate contraction may 

 be obtained on heating for each of the proteins said to exist in 

 the muscle fiber, f More recent observations J seem to show 

 that when a frog's muscle is gradually heated, only two really 

 distinct contractions are obtained, one at 39 C. (38 to 40) 

 or slightly lower, and one at 50 C. (49 to 51). Mammalian 

 muscle gives also two contractions when heated, one at 47 C. 

 (46 to 50) and one at 62 C. (61 to 64). In each of these 

 cases the second contraction is due to the action of heat on the 

 connective-tissue elements of the muscle. The first contraction is, 

 therefore, the one that is characteristic of the muscular substance 

 proper and the one that marks the occurrence of heat rigor. 

 At the tempertures stated, 39 C. for frog's muscle and 47 C. 

 for mammalian muscle, the viscous material within the sarco- 

 lemma coagulates. It does not follow necessarily that this coagula- 

 tion is the direct cause of the shortening. Meigs states that 

 plain muscle heated to 50 C. lengthens instead of shortening, 



1899 191, p. 127; also Inagaki, "Zeitschrift f. Biol.,"' " " ~~~ * 



Vrooman, " Bio-chemical Journal," 1907, 2, 363. 



Meigs, "American Journal of Physiology," 24, 1 and 178, 1909. 



