70 HUMAN PHYSIOLOGY 



that the blood in passing through the muscles has gained 

 carbon dioxide and lost oxygen. If the muscles be kept 

 contracted, it is further ascertained that the amount of 

 carbon dioxide gained is increased, while usually the amount 

 of oxygen taken up is also increased. This observation thus 

 confirms the investigations on the changes in the air sur- 

 rounding a muscle. 



But the solid constituents of the blood are also changed. 

 If the muscles have been contracting, the blood is found to 

 contain sarcolactic acid probably combined with ammonia. 



We shall afterwards find that blood contains small 

 quantities of glucose, C 6 H 12 6 . As it passes through 

 muscle it loses some of this, even when the muscle is at 



,GLUC05 E 

 FAT 

 OXYGEN 



SARCOLACTIC ACID 

 CARBON DIOXIDE 



FIG. 31. Exchanges between muscle and blood. 



rest, and a much larger amount when the muscle has been 

 active. 



The changes in the proteids of the blood going to and 

 coming from muscle have not been properly investigated. 



Some observers have obtained results which seem to 

 indicate that the amount of fat which is found in the blood 

 is diminished as the blood passes through the muscles, but 

 whether this diminution is greater during muscular activity 

 has not been studied. 



Such direct observations on muscle and the blood nourish- 

 ing it indicate that constant chemical changes are going on 

 when the muscle is at rest. It is constantly giving off carbon 

 dioxide and constantly consuming oxygen, glucose, and pos- 

 sibly fats and proteids. When doing work these chemical 

 changes become more active. We may compare resting 



