THE CHEMISTRY OF MUSCLE. 67 



being changed to sugar, undergoes eventually 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 increased in quantity by muscular activity. At- 

 tention 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 litmus paper, but per- 

 haps more strikingly by the use of acid fuchsin.* If a solution of 

 acid fuchsin is injected under the skin of a frog it is gradually ab- 

 sorbed and distributed to the body without injuring the tissues. 

 In the normal media of the body this solution remains colorless or 

 nearly so. If now one of the legs is tetanized the muscles take on a 

 red color, showing that an acid is produced locally. In the case of 

 the frog's muscle it is stated f that in the resting condition its 

 reaction expressed in terms of hydrogen ion exponent is equal to 

 pH 7.43, about the alkalinity of blood. When stimulated to the 

 point of fatigue the reaction changes to the acid side, pH = 6.84. 

 Experiments have been made by a number of observers to deter- 

 mine quantitatively the amount of lactic acid in the resting and the 

 worked muscle respectively. The most satisfactory results have 

 been obtained by Fletcher and Hopkins. J These observers have 

 shown in the first place that injury to a muscle causes a produc- 

 tion of lactic acid, and that, therefore, the usual method of deter- 

 mining the amount of this substance in supposedly resting muscle 

 has given fallacious results owing to the injury inflicted during the 

 process of extraction. By the adoption of a new method they have 

 avoided this error, and they find that in resting muscle lactic acid 

 exists in traces only (0.03 per cent.) or perhaps is absent altogether. 

 An appreciable amount is formed when the excised muscle is 

 well tetanized (0.22 per cent.), also after injury, and especially 

 in the development of rigor. In heat-rigor a maximum yield 

 of 0.3 to 0.5 per cent, is obtained in the frog's muscle. In a 

 muscle removed from the body and deprived, therefore, of its 

 supply of oxygen, lactic acid develops rapidly, reaching finally 

 an amount equal to that observed in heat-rigor. As long as 

 such a surviving muscle shows irritability toward artificial stim- 

 ulation, lactic acid continues to form. When irritability is lost, 

 no further production of acid can be detected and the muscle 



* Dreser, " Centralblatt fiir Physiologic," 1, 195, 1887. 



fPechstein, " Biochemische Zeitschrift," 68, 140, 1915. 



j Fletcher and Hopkins, "Journal of Physiology," 1907, 35, 247; also 

 1911, 12, 43, 286, and Embden et al., "Biochemische Zeitschrift," 1912, 45, 

 45. 



