66 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



of lactic acid on the fixed carbonates in the tissues. This liberation 

 of CO 2 has no special physiological significance. On the other 

 hand, when a muscle is made to contract in air or in oxygen, there 

 is a relatively large and sudden output of CO 2 which is obviously 

 dependent upon processes of oxidation in the muscle and is at- 

 tended by a parallel liberation of heat. This latter yield of CO2 

 does not take place if the muscle is made to contract under anaero- 

 bic conditions; that is to say, in the absence of oxygen. It would 

 seem to follow from this fact that the contraction or shortening of 

 the muscle is what may be called an anaerobic phenomenon, not 

 dependent upon oxidative changes. An oxidative reaction with a 

 resulting output of CO 2 follows the contraction under normal con- 

 ditions. Its significance in relation to the contraction will be re- 

 ferred to below. These facts seem to be in accord with prevalent 

 ideas regarding the nature of the muscular metabolism, according 

 to which the chemical processes take place in two stages. In the 

 first the complex energy-yielding material, sugar, for example, 

 undergoes a splitting process which results in the formation of 

 intermediary products, such as lactic acid. In the second stage 

 these intermediary products or some of them are oxidized, provided, 

 as Fletcher points out, there is an adequate supply of oxygen. 

 Under normal conditions a sufficient amount of oxygen is fur- 

 nished by the circulating blood, but under pathological conditions 

 and in the excised muscle, when air is excluded, the supply may not 

 be adequate, and as a result the intermediary products are not 

 oxidized completely. Under such conditions less heat is produced 

 in the muscle, and the intermediary products accumulate in the 

 tissue. 



The general views here stated are also in accord with the facts 

 regarding heat production in muscle during contraction, as stated 

 on p. 37. The so-called delayed heat production that occurs after 

 the mechanical process of shortenng is past is probably caused by 

 the oxidation of the intermediary products, the lactic acid perhaps, 

 and Hill* has shown that when oxygen is excluded, this second 

 phase in heat production drops out. 



Disappearance of the Glycogen. Satisfactory proof has been fur- 

 nished that the amount of carbohydrate in a muscle disappears 

 more or less in proportion to the extent and duration of the con- 

 tractions, and that after prolonged 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 next paragraph and in the section on 

 Nutrition. It is, perhaps, the general belief in physiology today 

 that under normal conditions the glycogen of the muscle, after 

 * Hill, "Journal of Physiology," 46, 28, 1913. 



