I 



CHEMICAL CHANGES DURING CONTRACTION. 455 



element has been described, and some authors state that the 

 latter increases at the expense of the former after an interme- 

 diate period in which the two substances seem fused together. 



Chemical Changes. During the contracted condition the chem- 

 ical changes which go on in passive muscle are intensified, and 

 certain new chemical decompositions arise, of which, however, 

 not much is known. 



Active muscle takes up more oxygen than muscle at rest, as is 

 shown by the facts that, during active muscular exercise, more 

 oxygen enters the body by respiration, and the blood leaving 

 active muscles is poorer in oxygen than when the same muscles 

 are passive. This absorption of oxygen cannot be detected in a 

 muscle cut out of the body, nor is any supply of oxygen neces- 

 sary for a contraction of such a muscle, since a frog's muscle will 

 contract in an atmosphere containing no oxygen. From this it 

 would appear that a certain ready store of oxygen must exist in 

 some chemical constituent of the muscle substance; and it is pos- 

 sible that some chemical compound, which is constantly renewed 

 by the blood existing in the muscle, is its normal source of oxygen, 

 and not the oxyhaemoglobin of the blood. 



The amount of CO 2 given off by a muscle increases in its state 

 of activity, as may be seen by the greater elimination from the 

 lungs during active muscular exercise, and by the fact that the 

 venous blood of a limb, when the muscles are contracted, con- 

 tains more CO 2 than when they are relaxed. The increase of CO 2 

 can also be detected in a muscle removed from the body and kept 

 in a state of contraction. Moreover, this increase in the forma- 

 tion of CO 2 in a muscle takes place whether there is a new supply 

 of oxygen given to it or not, and the quantity of CO 2 given off 

 always greatly exceeds the quantity of oxygen that is used up. 

 So that it is not exclusively, if at all, from the newly-supplied 

 oxygen that the CO 2 is produced. 



Muscle tissue, when passive, is neutral or faintly alkaline ; 

 during contraction, however, it becomes distinctly acid. The 

 litmus which it changes from blue to red is permanently altered, 

 and we can, therefore, conclude that CO 2 is not the only acid that 

 makes its appearance. The other acid is sarcolactic acid, which 



