THE CHEMISTRY OF MUSCLE. 67 



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 condi- 

 tions a sufficient amount of oxygen is furnished by the circulating 

 blood, but under pathological conditions and in the excised 

 muscle, especially when air is excluded, the supply may not be 

 adequate, and as a result the intermediary products are not oxi- 

 dized 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 in accord with the facts re- 

 garding heat production in muscle during contraction, as stated on 

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

 mechanical process of shortening 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. 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 propor- 

 tion to the extent and duration of the contractions, 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, per- 

 haps, the general belief in physiology to-day that under normal 

 conditions the glycogen of the muscle, after being changed to 

 sugar, undergoes a conversion to lactic acid as a result of the 

 stimulus that induces contraction, and that this formation of 

 lactic acid is directly or indirectly connected with the process of 

 shortening. 



The Formation of Lactic Acid. The lactic acid that is present 

 in the muscle is believed to be increased in quantity by muscular 

 activity. Attention was first called to this point by du Bois- 

 Reymond, who showed that the reaction of the tetanized muscle 

 is distinctly acid, while that of the resting muscle is neutral or 

 slightly alkaline. This fact can be demonstrated by the use of 



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



