THE CHEMISTRY OF MUSCLE. 65 



in the relative amounts of these salts cause marked changes in 

 the properties of the tissues, as is explained in the section on 

 Nutrition, in which the general nutritive importance of the salts 

 is discussed, and also in the section dealing with the cause of the 

 rhythmical activity of the heart. 



Chemical Changes in the Muscle during Contraction and 

 Rigor. The chemical changes known to occur in muscle as a result 

 of contraction under normal conditions are the following: (1) 

 Production of carbon dioxid. (2) Production of lactic acid. (3) 

 Disappearance of glycogen. A complete theory of contraction 

 would include, of course, ah explanation of the relation of these 

 changes to one another and to the processes of contraction and 

 relaxation. We do not at present possess such a theory, and 

 consequently the significance of the facts known can only be 

 guessed at or stated in a provisional way. 



Production of Carbon Dioxid. After increased muscular ac- 

 tivity it may be shown that an animal gives off a larger amount 

 of carbon dioxid in its expired air. In such cases the carbon dioxid 

 produced in the muscles is given off to the blood, carried to the 

 lungs, and then exhaled in the expired air. Pettenkofer and Voit, 

 for instance, found that during a day in which much muscular 

 work was done a man expired nearly twice as much CO2 as during 

 a resting day. The same fact can be shown directly upon an iso- 

 lated muscle of a frog made to contract by electrical stimulation. 

 The carbon dioxid in this case diffuses out of the muscle in part 

 to the surrounding air, and in part remains in solution, or in 

 chemical combination as carbonates, in the liquids of the tissue. 

 It has been shown by Hermann* and others that a muscle that has 

 been tetanized gives off more carbon dioxid than a resting muscle 

 when their contained gases are extracted by a gas pump. This CC>2 

 arises from the oxidation of some of the constituents of the muscle, 

 and its existence is an indication that in their final stages the 

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

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

 Fletcher f has discovered the significant fact that the increased 

 elimination of CO 2 following upon contraction is clearly shown only 

 when the muscle is well supplied with oxygen. In the absence of 

 oxygen contraction may cause no increase in the CO 2 given off. 

 This author calls attention to the fact that CO 2 is given off by a 

 muscle in two ways. . In the resting excised muscle there is a con- 

 stant small formation of C0 2 which he believes is due to the action 



* Hermann, " Untersuchungen iiber den Stoffwechsel der Muskeln, etc.," 

 Berlin, 1867. 



t Fletcher, "Journal of Physiology," 1902, 28, 474; also Fletcher and Hop- 

 kins, Croonian Lecture, "Proceedings of the Royal Soc.," London, B. 89, 444, 

 1917. 



