50 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



rigor, which in this case is of a more feeble character and shorter 

 duration. The development of rigor is very much hastened by many 

 drugs that bring about the rapid death of the muscle substance, such 

 as veratrin, hydrocyanic acid, caffein, and chloroform. A frog's mus- 

 cle exposed to chloroform vapor goes into rigor at once and shortens 

 to a remarkable extent. Rigor is said also to occur more rapidly 

 in a muscle still connected with the central nervous system than 

 in one whose motor nerve has been severed. After a certain 

 interval, which also varies greatly, from one to six days in human 

 beings, the rigidity passes off, the muscles again become soft and 

 flexible; this phenomenon is known as the release from rigor. In 

 the cold-blooded animals the development of rigor is very much 

 slower than in warm-blooded animals. Upon an isolated frog's 

 muscle the most striking fact regarding rigor mortis is the shortening 



Fig. 24. 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 intervals of six hours. It will be seen that the shortening 

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



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, 

 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 proteids 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 

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



