THE PHENOMENON OF CONTRACTION. 53 



that the muscle undergoes. This shortening or contraction comes 

 on slowly, as is shown in the accompanying figure, but in extent 

 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. 

 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,* who proved that the 

 muscle plasma is really coagulable. After first freezing and mincing 



Fig. 25. Curve of normal rigor mortis, gastrocnemius muscle of frog. The curve 

 was obtained upon a kymographion making one revolution in eight days. The marks on 

 the line below the curve indicate interval', of six hours. It will be seen that the shortening 

 required eighteen hours, the relaxation about seventy-two hours. 



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 



* Kiihne, "Archiv f. Physiologic," 1859, p. 788. 



