364 VETERINARY PHYSIOLOGY 



In the literature of aniiiiiil nutrition the term "digesti- 

 bihty " is used in a specific sense. It denotes the percentage 

 of the food or of any constituent of the food that is absorbed 

 from the aUmentary tract. Thus in a (Ugestion experiment 

 on a horse, 57 per cent, of the protein of hay was apparently 

 digested and absorbed as that proportion was not recovered 

 in the faeces. The "digestibility" of the protein of hay for 

 the horse in this case is said to be 57 per cent. The per- 

 centage of digestibility is often termed the " coefficient of 

 digestibility." 



Digestion Experiments. 



The availability of food-stuffs is determined by digestion 

 experiments. In these the animal is fed on a weighed 

 quantity of the food to be tested for a preliminary period 

 of about ten days to make sure that the previous food has com- 

 pletely passed out of the intestinal tract. The feeding is 

 then continued for another period of not less than ten days, 

 during which time twenty-four hourly collections of faeces 

 are made. The ditference between the amount of each 

 constituent of the food eaten and that found by analysis in 

 the faeces is regarded as the digested portion. 



Concentrates cannot be fed alone to ruminants. Their 

 availability is determined by superimposing a weighed 

 quantity upon a roughage diet, whose availability has been 

 previously determined. The increased amount of the various 

 constituents found in the faeces is refiarded as the undigested 

 matter of the concentrate tested. 



Accuracy of the Method. — Digestion experiments, though 

 of great use, indicate only apparent digestibility. The issue 

 is confused by (1) excretory products ; and (2) loss through 

 fermentation. 



(1) Nitrogenous substances, ether soluble substances, and 

 inorganic salts are present in the faeces even if absent in the 

 food (p. 3G2). The smaller the amount of these present in 

 the food the greater is the percentage error due to excretory 

 products ; with fats and salts the error ma}^ be so great as 

 to make the results of little value as a means of determining 

 the real amount digested. 



