CH, XII.] CHEMICAL CHANGES IN MUSCLES 131 



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. Sarcolactic 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. 



When a muscle contracts, the quantity of oxygen consumed is 

 increased, and the discharge of carbonic acid is increased also. This 

 is the cause of the rise in heat-production just described. It is 

 important to note, however, that this change follows rather than 

 accompanies the increase of work, as we shall see more at length 

 when we are studying tissue-respiration (p. 397). 



For a certain time after its removal from the body, an excised 

 muscle can be made to contract and give off oxidation products such 

 as carbonic acid in an atmosphere containing no oxygen at all. The 

 oxygen used has thus been 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. Excised muscles, however, must be regarded as partially 

 asphyxiated, for their individual fibres are largely cut off from that 

 ready supply of oxygen which normally reaches them by the blood. 

 During life (and the living condition can be imitated by placing an 

 excised muscle in an atmosphere of pure oxygen) the muscular 

 substance breaks down into a number of simpler substances ; one 

 of these is carbonic acid. The others, however, or some of them, 

 are at once built up again with the inclusion of oxygen and some 

 carbon-containing substance, perhaps sugar, into living material. 

 The muscle, therefore, does not contain any of the by-products of 

 its own metabolism. In excised muscle, when the oxygen supply is 

 deficient the by - products accumulate, as a result of which very 

 striking alterations take place : (1) The reaction of the muscle 

 changes and the phenomena of fatigue and functional death set in. 

 (2) The proteins become coagulated, and this is the physical basis of 

 rigor mortis. 



There are other chemical changes in the muscle when it contracts, 

 for instance, a change of glycogen into sugar. The question whether 

 nitrogenous waste is increased during muscular activity has been a 

 much debated one. It has now, however, been finally proved that 

 an increased consumption of carbon (in large measure derived from 

 the carbohydrate stored in the muscle) is a marked and immediate 

 feature of muscular activity. Any increase in the consumption of 





