THE CHEMISTRY OF MUSCLE. 67 



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. 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 shghtly alkaline. This fact can be 

 demonstrated by the use of litmus paper, but perhaps more strik- 

 ingly by the use of acid fuchsin.* If a solution of acid fuchsin is in- 

 jected under the skin of a frog it is gradually absorbed and distrib- 

 uted 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 t that in the resting condition its reaction expressed in terms of 

 the hydrogen ion exponent is equal to pH 7.43, about the alkalinity 

 of blood. For a definition of hydrogen exponent, see p. 414. 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 ob- 

 servers to determine quantitatively the amount of lactic acid in the 

 resting and the worked muscle respectively. The most satisfactory 

 results have been obtained by Fletcher and Hopkinsf. These ob- 

 servers have shown in the first place that injury to a muscle causes a 

 production of lactic acid, and that, therefore, the usual method of 

 determining the amount of this substance in supposedly resting 

 muscle has given fallacious results owing to the injury inflicted dur- 

 ing 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 es- 

 pecially 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 fur Physiologie," 1, 195, 1887. 



t Pechstein, "Biochemische Zeitschrift," 68, 140, 1915. 



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

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

 45, 45. 



