88 PHYSIOLOGY OF MUSCLE AND NERVE 



followed by (a) a greater production of carbon dioxid and a greater con- 

 sumption of oxygen, (b) a formation of lactic acid, and (c) a gradual 

 disappearance of the glycogen. Hence, as the contraction of a muscle 

 is made possible by chemical alterations in the myoplasm, it must be 

 evident that this energy is chiefly derived from the carbohydrates. 

 The statement that this foodstuff is the most available source of 

 muscular energy, is substantiated further by the fact that muscular 

 exercise immediately raises the respiratory quotient. 



The production of carbon dioxid by the active muscles is clearly proved by the 

 fact that the expired air contains a larger amount of carbon dioxid than the 

 inspired. Obviously, this gas is transferred from the tissues to the blood and 

 is subsequently gotten rid of through the respiratory channel. It has also been 

 shown that an excised muscle evolves a much larger quantity of carbon dioxid 

 when tetanized than when allowed to rest.^ This increased production of carbon 

 dioxid is associated with an increased intake of oxygen, but the respiratory quo- 



CO 



tient, —p—^' increases, because the output of carbon dioxid exceeds the absorption 



O2 

 of oxygen. Moreover, this evolution of carbon dioxid ceases if no oxygen is al- 

 lowed to enter the body. In explanation of these phenomena it has been stated 

 that this gas does not constitute a primary product, but arises secondarily in con- 

 sequence of the oxidation of the products of muscular metabolism. 2 Thus, it has 

 been assumed that the chemical processes in muscle result first of all in a decom- 

 position of the complex nutritive material into intermediary substances, such as 

 lactic acid, which are then reduced in the presence of an adequate supply of oxygen. 



This explanation finds substantiation in the fact that frog's muscle, when sus- 

 pended in an atmosphere of nitrogen, soon ceases to respond to stimulation. If it 

 is then subjected to an analysis, it will be found to contain 0.2 per cent, of lactic 

 acid, but only traces of carbon dioxid. The latter, in all probability, have been 

 liberated in consequence of the change of the muscle medium from faintly alkaline 

 to acid. Conversely, if a muscle is first fatigued in an atmosphere of nitrogen, 

 and is then transferred into a medium of pure oxygen, it soon recovers its irritability 

 and may be stimulated for a long time before it again exhibits indications of 

 fatigue. On subsequent analysis, it will be found to contain practically the same 

 amount of lactic acid as resting muscle, but much larger quantities of carbon 

 dioxid. A contracting muscle, therefore, liberates carbon dioxid in amounts 

 which are almost directly proportional to the quantity of oxygen available for the 

 reduction of the lactic acid. 



The Formation of Lactic Acid. — Resting muscle exhibits a neutral or feebly 

 alkaline reaction, while active muscle is distinctly acid.^ This general statement, 

 as we have just seen, holds true only if inconsiderable amounts of oxygen are 

 available, because a copious supply of this gas reduces the sarcolactic acid still 

 further, while a scarcity of it causes the acid to accumulate. But, since mechanical 

 manipulation and thermal and chemical irritations are very prone to increase the 

 production of this acid, it is difficult to obtain an excised muscle with a perfectly . 

 neutral reaction.^ In most cases it will show an acidity equalling 0.02 per cent., 

 expressed as zinc lactate. This may be considerably increased (0.2 per cent.) by 

 causing the muscle to undergo a few contractions. Blue litmus paper will then be 

 reddened and brown turmeric paper turned yellow. 



The production of lactic acid during muscular activity may be proved by inject- 

 ing a solution of acid fuchsin into the dorsal lymph sac of a frog, whence it will be 



1 Hermann, Unters. iiber d. Stoffwechsel d. Muskeln., Berlin, 1867. 



2 Fletcher, Jour, of Physiol., xxviii, 1902, 474. 

 ' Proved by DuBois-Reymond, in 1859. 



* Fletcher and Hopkins, Jour, of Physiol., xxxv, 1907, 247; and xliii, 1911, 12. 



