THE NUTRIENTS AND NUTRITION 319 



percentage of a nutrient which is digested is called the 

 digestive coclKcient of that nutrient. The digestive coeffi- 

 cients vary for each nutrient and are determined l^y direct 

 experiment. 



These experiments, which are usually referred to as diges- 

 tion trials, are made as follows: A fowl is fed a given 

 amount of feed, the exact composition of which has Ijeen 

 determined by analysis. All the intestinal voidings pro- 

 duced during the period are carefidly collected, weighed, and 

 analyzed, the excreta from the kidneys having been diverted. 

 The undigested portions of the nutrients appear in the 

 voidings, and the difTerence between the amount fed and that 

 excreted, reduced to a percentage basis, represents the 

 digestion coefficient. It is never exactly accurate, however, 

 since some waste material is given off from the intestines. 



Digestion trials are very much more difficult with poultry 

 than with other farm animals, owing to the fact that the 

 urine is not temporarily stored in the bladder and eliminated 

 through a separate genito-urinary opening, as in mammals, 

 but is conveyed directly from the kidneys through the ureters 

 to the cloaca, where it is constantly mixed with the fecal 

 matter from the intestines. "The fact that the urine and 

 feces are excreted together has formed the chief obstacle to 

 progress in the performance of digestion experiments with 

 poultry."' This is because the so-called urine contains the 

 broken-down tissue from the body, which cannot be dis- 

 tinguished from the undigested portions of the food by 

 present analytical methods. 



The white, pasty material appearing in the droppings of 

 birds is uric acid, excreted by the kidneys. This increases 

 in amount as the ration becomes more nitrogenous. The 

 tlroppings of wild birds living almost entirely upon worms and 

 insects are quite white. 



No satisfactory method of separating the urine from the 

 feces has been generally adopted. Some successful work 

 has been done by means of a surgical operation, whereby a 

 false urinary aperture has been made. In very few cases 



' Brown, Bureau of Animal Industry, Bulletin No. 56. 



