THE PHENOMENON OF CONTRACTION. 53 



animals or soldiers killed in battle, is usually followed quickly by 

 muscle rigor; indeed, in extreme cases it may develop almost imme- 

 diately. Death after wasting diseases is also followed by an early 

 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. 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 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. 

 With regard to the specific cause of the contraction of death rigor 

 the most significant fact is the discovery made by Fletcher and 

 Hopkins,* that when an excised muscle is kept well supplied with 

 oxygen it dies without giving a rigor contraction. The presence of 

 oxygen ensures the oxidation of the lactic acid and thus prevents its 

 accumulation in the dying muscle. We may infer on the basis of 

 this result that the contraction in death rigor is due to the accumu- 

 * Fletcher and Hopkins, "Journal of Physiology," 1907, 35, 247. 



