THE TISSUES 71 



inuscle as to 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 very 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 

 this 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. 



If this precaution is taken, it is found that the excretion 

 of the various elements composing muscle is modified by 

 muscular work. 



Attention has chiefly been directed to the variations in the 

 carbon and nitrogen thrown off, the former mainly as carbon 

 dioxide in the expired air, the latter as urea in the urine. 

 It has been found that if a fasting or underfed animal is 

 made to do work, the excretion of both these elements is 

 increased, the carbon proportionately to the work done, 

 the nitrogen in quantities not strictly proportionate to 

 the work, being greater the more underfed the animal 

 is and the harder the work done, and being less the better 

 nourished the animal is or the less the work that is done. 

 (Fig. 32, 1). 



If a lean animal be fed on an exclusively proteid diet, the 

 excretion of carbon and nitrogen is increased, practically 

 proportionately to the work done (Fig. 32, 2). 



But if the animal be well fed on an ordinary diet, contain- 

 ing proteids, carbohydrates, and fats, the performance of 

 muscular work increases the excretion of carbon proportion- 



