286 DIGESTIBILITY OF FOODS 



this preliminary period should be from 6 to 8 days ; with pigs, 4 to 6 

 days are sufficient. The experiment proper, during which all the food, 

 of known composition, is carefully weighed and all solid excrement 

 both weighed and analysed, should also last for at least a week or 10 

 days so as to eliminate accidental variations. 



Even with the most careful attention to accuracy in such experi- 

 ments, the results are not entitled to any great reliance, for there are 

 several factors which invalidate their correctness. The dung does not 

 consist entirely of undigested food, but contains some residues from 

 the digestive fluids and which therefore have undergone digestion and 

 are products of metabolism in the same way as the constituents of the 

 urine. These secretions increase the nitrogenous and fatty constitu- 

 ents of the faeces and thus tend to give too low values for the diges- 

 tibility of the proteids and fats of the food supplied. 



The proportion of each food constituent digested out of 100 

 supplied, is known as the " digestion coefficient ". It is to be noted 

 that what is really determined by experiments of the kind just dis- 

 cussed is the difference between the total food and the fasces, for which 

 the term availability has been proposed, while true digestibility 

 is the difference between the total food and the undigested residue. 

 For a determination of the lattef, no accurate method has yet been 

 devised, because of the impossibility of distinguishing between the 

 undigested residue of the food and the matter derived from digestive 

 secretions. 



The digestibility or availability of a food has no reference to the 

 ease or rapidity with which it can be assimilated, nor to any effect it 

 may have upon the health or comfort of the animal receiving it. These 

 are points on which the aroma and flavour of the food and the in- 

 dividuality of the animal have more influence than the true digestibility. 

 Flavour, aroma and palatability are factors, which, though almost im- 

 possible to measure, are of great practical importance in the feeding of 

 animals. They often depend upon apparently slight differences in the 

 conditions under which the food has been prepared or stored. Such 

 apparently trivial circumstances as absence or presence of slight mouldi- 

 ness, faint taints with distasteful flavours, etc., while without appreciable 

 effect upon either the composition or digestibility of the food, have 

 often a marked influence upon its success in fattening animals. Foods 

 which are eatea with relish are almost invariably more successful than 

 similar foods, which, from any cause, have become distasteful. 



The digestibility of a given food differs considerably with different 

 kinds of animal and even with different individuals of the same breed. 



The ruminants, by virtue of their more thorough mastication and 

 longer alimentary canal, are better able to digest coarse fibrous foods 

 than the horse or the pig. The difference becomes less with the more 

 concentrated foods and is most marked when the crude fibre of the food 

 is alone considered. 



Gentle exercise rather increases digestibility but hard work dis- 

 tinctly lowers it. This has been clearly shown by the exhaustive ex- 

 periments of Grandeau and his associates with horses. 



The quantity of food supplied to an animal has little influence on 



