66 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



living muscle it would serve very well to explain the production 

 of lactic acid. From experiments made in general nutrition it 

 has been shown that in birds especially the uric acid in the urine 

 is replaced largely by lactic acid (ammonium lactate) when the liver 

 is excised. Under these conditions the quantity of lactic acid 

 secreted varies with the albumin destroyed in the body, and many 

 physiologists are of the opinion that the lactic acid produced in 

 the muscle or in other tissues is derived from the breaking down 

 of the living proteiii material. On the other hand, a study of the 

 action of the enzymes present in muscle leads to the other conclu- 

 sion, namely, that the lactic acid arises from a splitting of the 

 sugar. (Consult section on nutrition.) 



The Formation of Creatin. Creatin constitutes the chief nitrog- 

 enous waste product in the muscle, and we should expect that the 

 greater metabolism during activity would result in an increase in 

 the creatin. Some observers state positively that the creatin is 

 increased during contraction. 



Chemical Changes during Rigor Mortis. The chemical 

 changes during rigor have been referred to above, but may be 

 summarized here in brief form: 



1. There is a coagulation of the proteid material of the muscle 

 plasma, which at present is explained by supposing that the con- 

 tained myosin and myogen, spontaneously, or under the action of 

 an enzyme, pass into their insoluble forms, namely, myosin fibrin 

 and myogen fibrin. 



2. There is an increased acidity, due doubtless to a production 

 of lactic acid. 



3. There is a production of CO 2 . Hermann, in his original ex- 

 periments, asserts that in rigor there is, so to speak, a maximal 

 production of CO 2 , that is, all of the material in the muscle capable 

 of yielding CO 2 is broken down during rigor. The amount of CO 2 

 given off, therefore, by a resting muscle when it goes into rigor 

 is greater than in the case of a worked muscle, since in the 

 latter some of the material capable of yielding CO 2 has been used 

 up during contraction. 



4. The consumption of glycogen. According to some observers, 

 glycogen disappears during rigor as it does during contraction; 

 but others find that the amount is not changed during this process. 

 As the glycogen after death is converted to sugar with some rapidity 

 it is possible that the disappearance noted by the former observers 

 was not due to the rigor process, but to postmortem fermentation. 



The Relation of the Chemical Changes during Contraction 

 to Fatigue ; Chemical Theory of Fatigue. As we have seen, a 

 muscle kept in continuous contraction soon shows fatigue; it 

 relaxes more and more until, in spite of constant stimulation, it 



