68 



VETERINARY PHYSIOLOGY 



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 

 output of carbon and nitrogen, the two most important elements 

 in muscle, the former mainly appearing 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. 33, i). 



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



FIG. 33. To illustrate the influence of Muscular Work upon the Excretion of 

 Carbon and of Nitrogen (1) in a fasting or underfed animal ; (2) in an 

 animal fed on proteins ; (3) in an animal on a normal diet. 



excretion of carbon and nitrogen is increased, practically 

 proportionately to the work done (fig. 33, 2). 



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

 ing proteins, carbohydrates, and fats, the performance of 

 muscular work increases the excretion of carbon proportion- 

 ately to the work done, but may cause only a very slight 

 increase in the excretion of nitrogen (fig. 33, 3). 



From the increased excretion of nitrogen and carbon the 

 consumption of proteins may be calculated, since proteins 

 contain 16 per cent, of nitrogen and 52 per cent, of carbon 

 i.e. 3'4 times more carbon than nitrogen. Each gram of 

 nitrogen excreted thus represents the breaking down of 6'25 

 grams of protein, and it is accompanied by 3*4 grams of 

 carbon. If more carbon is excreted, it must come from 

 carbohydrates- or fat. 



