DIGESTION AND DIGESTIBILITY 27 



The second, and only useful, method, then, is to deter- 

 mine the digestibility of the food in the particular 

 animal desired by giving a known weight of the food, 

 and then collecting and estimating the amount of 

 undigested waste matter in the excreta. To find the 

 digestibility of a particular food for the horse, the pro- 

 cedure consists in giving the food for several days before 

 commencing records of the experiment, in order that the 

 digestive tract may be emptied of all but that food when 

 the collection and estimation of the excreta commences. 

 After this, for six or seven days the animal receives daily 

 a known weight of the food, and by means of a specially 

 arranged bucket all the dung is caught. The droppings 

 for the day are all mixed together, and a certain weight 

 is taken for analysis. 



Knowing the chemical composition and weight of the 

 food supplied, and also the chemical composition and 

 weight of the excreta, it is easily seen, by difference, how 

 much of the food was digested and absorbed for every 

 100 parts supplied. As stated above, the carbo-hydrates, 

 proteins, and fats, in a food have to undergo different 

 changes in the digestive tract before they can be 

 absorbed, so that it is not surprising to find that even 

 in the same food these nutrients are digested to quite 

 different extents. In one food the fats may be in a very 

 digestible form, whilst the proteins are digested only 

 with difficulty and to a small extent ; in another food 

 the reverse may be the case. Thus it is necessary to 

 determine by experiment with each food what proportion 

 of each of its nutritive components is digested by the 

 animal in question — in this case the horse. In the case of 

 a food which it is impossible or inconvenient to give alone 

 (such as any of the straws), it is fed along with some 



