66 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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

 of COo 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 CO2 which is obviously 

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

 tended by a parallel lil^eration 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. The oxida- 

 tive reaction with its resulting output of CO2 takes place after the 

 contraction is over under normal conditions. These facts seem to 

 be in accord with prevalent ideas regarding the nature of the mus- 

 cular metabolism, according to which the chemical processes take 

 place in two stages. In the first the complex energy-yielding mate- 

 rial, sugar, for example, undergoes a splitting process which results 

 in the formation of intermediary products, such as lactic acid. Part 

 of the energy of this process, as we have seen, is converted to me- 

 chanical energy which causes the shortening of the muscle. In the 

 second stage these intermediary products or some of them are oxi- 

 dized, provided, as Fletcher points out, there is an adequate supply 

 of oxygen. Part of the energy yielded by this second process is util- 

 ized probably in restoring the muscle to its previous condition, that 

 is to say, it is stored in the muscle in available potential form. 

 Under normal conditions a sufficient amount of oxygen is furnished 

 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 contraction is over 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. It 

 is generally assumed that under normal conditions the glycogen of 

 the muscle, after being changed to sugar, undergoes eventually a 

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



