THE TISSUES 67 



diminished as the blood passes through the muscles, but 

 whether this diminution is greater during muscular activity 

 has not been studied. 



. 



CARBON DIOXIDE 



FIG. 32. Exchanges between muscle and blood. 



Such direct observations on muscle and the blood nourishing 

 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 possibly fats 

 and proteins. When doing work these chemical changes 

 become more active. We may compare resting muscle in its 

 chemical changes to an engine with its fires banked down. 

 Active muscle is comparable to the engine with its fires in full 

 blast. 



4. Effects of Muscular Work upon the Excreta. Another 

 method of study has yielded results of very great value 

 the investigation of the effects of muscular work upon the 

 excreta. 



Not only is muscle the most bulky and most constantly 

 active tissue, but it is the tissue in which the most extensive 

 chemical changes occur in the liberation of the energy for 

 work and heat production ; and hence, the waste products of 

 the body are chiefly derived from muscle, and their amount 

 and character must afford an indication of the changes in 

 that tissue. 



This was long ago recognised, but the older experimenters 

 did not sufficiently realise that the excretions are modified 

 by the amount and character of the food taken, and hence 

 their results are of little value. In studying the influence 

 of muscular work on the excreta, food must be withheld or 

 must be unvarying during the experiment. 



