THE CHEMISTRY OF MUSCLE. 63 



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. The oxidation, 

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

 The 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, 

 and the same may be said of the functional activity of the kidney. 

 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 Gli/cogen. 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- 

 longed muscular activity, especially in the starving animal, the 

 supply may be exhausted entirely. In what way the glycogen is 

 consumed is not completely known; the matter is discussed in the 

 section on Nutrition. The most probable view is that the glycogen 

 is first converted to sugar (dextrose) by the action of an amylolytic 

 enzyme, and the sugar in turn is destroyed by the serial action of 

 several enzymes. The first step, probably, is a conversion to lactic 

 acid (C 6 H 12 O. == 2C 3 H 6 O 3 ), and the lactic acid then undergoes 

 oxidation, with the production of CO 2 and H 2 O, under the influence 

 of an oxidizing enzyme, either directly or after conversion to still 

 lower members of the fatty acid series (acetic or formic acid). 

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



