240 PHYSIOLOGY 



lactic acid or of C0 2 , whereat the formation of these bodies seems to 

 bear an important relation to the occurrence of rigor. Thus after 

 severe muscular fatigue, as in hunted animals, where there has already 

 been a considerable formation of these waste products of muscular 

 contraction, rigidity may come on almost immediately after death. 

 If living muscle be plunged into boiling water, it undergoes instant 

 coagulation, but no chemical change. The reaction of the scalded 

 muscle, like that of fresh muscle, is slightly alkaline to litmus. No 

 sarcolactic acid or carbonic acid is produced. On the other hand, in 

 surviving muscle, after the cessation of the circulation, there is a 

 steady formation of lactic acid which accumulates in the muscle. 

 The actual coagulation of the muscle -proteins occurring in rigor is 

 largely, if not entirely, determined by the increasing acidity of the 

 muscle thereby produced. In fact, it is the production of the acid 

 which causes the onset of rigor, and not the rigor which causes a sudden 

 formation of acid. Hence if the accumulation of lactic acid be pre- 

 vented by perfusing the muscle with salt solutions, the onset of rigor 

 may be postponed indefinitely, and the muscle may begin to putrefy 

 without having undergone rigor. 



The lactic acid formed in muscle (sarcolactic acid) is a physical isomer of 

 the lactic acid formed in the fermentation or souring of milk. They both 

 have the formula CH 3 .CH(OH).COOH, i.e. they are ethylidene lactic acids. 

 The lactic acid of fermentation is optically inactive ; sarcolactic acid rotates 

 polarised light to the right ; while a third isomer which is laevo-rotatory is 

 produced by the action of various bacilli and vibriones on cane sugar. 



THE CHEMICAL CHANGES WHICH ACCOMPANY ACTIVITY 

 The principle of the conservation of energy teaches us that the 

 energy of the contraction of muscle must be derived from chemical 

 changes, probably processes of decomposition and oxidation, occurring 

 in the muscle itself. In seeking out the nature of these changes three 

 methods are open to us : 



(1) We can examine the changes in the muscle itself, avoiding so 

 far as possible reintegrative changes by working on excised muscles. 



(2) We can investigate the changes in the medium surrounding the 

 muscle. Muscle may be exposed in a vacuum or in a confined space 

 of air, and its gaseous interchanges during rest and activity compared. 

 Or we may lead a current of defibrinated blood through excised muscles, 

 and determine the change in the composition of the blood before and 

 after passing through the muscle Under various conditions. 



(3) A method which, although apparently complex, has rendered 

 the utmost service to the physiology of muscle is to use the changes 

 in the total metabolism of the animal during rest and muscular work 

 as a clue to the muscular metabolism itself. In such a case the 



