THE PHENOMENON OF CONTRACTION. 51 



it exceeds the simple contraction obtainable from the living muscle 

 by means of a maximal stimulus. This part of the phenomenon 

 is, however, much less marked apparently in mammalian muscle, 

 and Folin* states that, if rigor be caused in frog's muscle by 

 lowering its temperature to 15 C., the muscle becomes rigid 

 merely without undergoing any shortening or change in translu- 

 cency. The usual explanation that is given of rigor is that it is 

 due to a coagulation of the fluid substance, the muscle plasma, of 

 which the fibers are constituted. During life the proteins exist in 

 a liquid or viscous condition;' after death they coagulate into a 

 solid form. This view is referred to again in the chapter dealing 

 with the chemistry of muscle and nerve; it has received much 

 support from the investigations of Kiihne,f who proved that the 

 muscle plasma is really coagulable. After first freezing and mincing 

 the muscles he succeeded in squeezing out the plasma from the 

 living fibers and showed that it subsequently clotted. While the 

 coagulation theory of rigor explains the greater rigidity of the 

 muscle, it does not furnish in itself a satisfactory explanation of 

 the shortening, and the fact, as stated above, that the rigidity 

 may occur without the shortening indicates that this latter process 

 may possibly be due to changes that precede the appearance of 

 rigidity. 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. According to some physiologists, the 

 processes may be regarded as the same, the heat rigor being simply 

 a death rigor that is rapidly developed 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 conditions, 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 accel- 

 erated 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 C4 According to other observers, heat rigor is due to an 

 ordinary heat coagulation of the proteins present in the muscle 



* " American Journal of Physiology," 9, 374, 1903. 



fKiihne, " Archiv f. Physiologie," 1859, p. 788. 



j JLatimer, " American Journal of Physiology/' 2, 29, 1899. 



