CH. XIII.] 



THE GASES OF MUSCLE 



149 



the other direction, indicating the production of heat first on one 

 side, then on the other. 



Chemical Changes in Muscles. 



The chemical changes which are normally occurring in a resting 

 muscle are much increased when it contracts. Waste products of 

 oxidation are discharged, and the most abundant of these is carbonic 

 acid. Sarco-lactic acid is also produced, and the alkaline reaction of 

 a normal muscle is replaced by an acid one. The muscles of animals 

 hunted to death are acid ; the acid reaction to litmus paper of a frog's 

 gastrocnemius can be readily shown after it has been tetanised for 10 

 to 15 minutes. 



The quantity of oxygen consumed is increased, but the con- 

 sumption of oxygen will not account for the much greater increase 

 in the discharge of carbonic acid. This is illustrated by the 

 following table : 



Indeed, a muscle can be made to contract and give off oxidation 

 products like carbonic acid in an atmosphere containing no oxygen 

 at all. The oxygen used is thus stored up in the muscle previously. 

 The oxygen is not, however, present in the free state, for no oxygen 

 can be detected in the gases obtained from muscles by means of an 

 air-pump. Hermann has supposed that the oxygen enters into the 

 formation of a complex hypothetical compound he calls inogen. On 

 contraction he considers this is broken up into carbonic acid, sarco- 

 lactic acid, and a proteid residue of myosin. 



There are other chemical changes in the muscle when it contracts 

 namely, a change of glycogen into sugar, and an increase of nitro- 

 genous waste. The question whether urea is increased during 

 muscular activity is, however, a much debated one, and we shall 

 return to it when we are studying the urine. What is certain is 

 that the increased consumption of carbon (possibly in large measure 

 derived from the carbohydrate stored in the muscle) is a much more 

 marked and immediate feature than an increase in the consumption 

 of nitrogen. 



