DIGESTION OF THE FOOD 27 



undigested part of the food is expelled as faeces 

 varies considerably. With cattle and sheep it 

 takes on an average three to four days, whilst with 

 pigs thirty-six hours suffice. 



(3) The determination of the digestibility of foods. 



To learn what substances in a food-stuff are 

 digested and taken up by the animal body, it is 

 usual to undertake digestibility experiments with 

 animals. For this purpose the food under examina- 

 tion is first carefully analysed and a weighed por- 

 tion given to the animal, generally a sheep ; after- 

 wards the dung is analysed and the quantity of 

 undigested material which it contains is determined. 

 As it is some time before the residues from the 

 previous feeding are expelled completely from the 

 digestive organs, the food to be tested must be 

 given for a period of some days before the analysis of 

 the faeces begins. Ruminants and horses are usually 

 allowed six to eight days and pigs four to six days. 



Owing to the differences in the composition of 

 the faeces, even when the supply and quality of the 

 food is constant, it is essential to collect them for 

 at least eight days when experimenting with oxen, 

 and at least six days for pigs. Each day an aliquot 

 part, say, one-tenth or one-fifth of the well-mixed 

 faeces, is taken, at once dried, and then after being 

 left exposed to the air to render it " air dry " it is 

 analysed in exactly the same way as the food. 



