THE CHEMISTRY OF MUSCLE. 63 



changes in the muscle are equivalent to those of ordinary combus- 

 tion at high temperatures, the burning of wood or fats, for instance. 

 Moreover, the formation of the CO 2 in the muscle is accompanied 

 by the production of heat, as in combustion; and for the same 

 amount of CO 2 produced in the two cases the same amount of heat 

 is liberated. It has been shown, however, in the frog's muscle 

 freshly removed from the body, that the CO 2 is produced whether or 

 not any oxygen is supplied to the muscle, that is, when the muscle 

 is made to contract in an atmosphere containing no oxygen, or 

 in a vacuum. In this respect the parallel between physiological 

 oxidation and ordinary combustion fails. Wood, oil, and other 

 combustible material cannot be burnt at high temperatures in 

 the absence of oxygen. We must believe, therefore, that in the 

 muscle there is a supply of stored oxygen, and that the muscle 

 will give off CO 2 as long as this supply lasts. One suggestion 

 that is made is that the oxygen is stored as intramolecular oxygen, 

 that is, the oxygen taken in by the muscle tissue while the blood 

 is circulating through it normally, is assimilated or combined by 

 the living molecules. When these molecules break down as a con- 

 sequence of the stimulus applied to the muscle the excess of oxygen 

 unites with some of the carbon to form the CO 2 . The oxidation, 

 instead of being direct, as in the case of combustions, is indirect. 

 This and other views regarding the nature of the oxidations in 

 the body are treated in the section on nutrition. 



The oxygen is absolutely necessary to the normal activity of 

 the muscular tissue, but the tissue, by storing the oxygen, can 

 function for some time when the supply is suspended. As Pfliiger 

 has expressed it, in a most interesting paper,* the oxygen is like 

 the spring to a clock : once wound up, the clock will go for a cer- 

 tain time without further winding. It must be borne in mind, 

 however, that different tissues show considerable variation in the 

 time during which they will function normally after suspension 

 of their oxygen supply. The cortex of the brain, for instance, 

 loses its activity, that is, unconsciousness ensues, almost imme- 

 diately upon cessation or serious diminution in the supply of blood. 

 In the cold-blooded animals, with their slower chemical changes, 

 the supply of stored oxygen maintains irritability for a longer 

 time than in the warm-blooded animals. 



Disappearance of the Glycogen. An equally positive chemical 

 change in the muscle during contraction is the disappearance of its 

 contained glycogen. Satisfactory proof has been furnished that the 

 amount of glycogen in a muscle disappears more or less in proportion 

 to the extent and duration of the contractions, and that after pro- 



* Pfliiger, "Archiv f. die gesammte Physiologic," 10, 251, 1875. 



