104 THE CHEMISTRY OF CATTLE FEEDING 



Concentrated foods, such as oats, oil-cakes, etc., cannot 

 be used alone, but must be fed to the animal along with hay 

 or other bulky fodder, the digestibility of which has been 

 previously determined. By subtracting the amount of protein 

 in the bulky fodder from the total amount in the ration con- 

 sumed, and subtracting the amount of protein corresponding 

 to it from the total amount in the faeces, the data are obtained 

 for calculating the digestibility of the protein in the con- 

 centrated food. 



The fact that the faeces contains effete bowel membranes 

 and other ingredients not directly derived from the food tends 

 to vitiate the results. No means is known by which this 

 difficulty can be overcome ; but the amount of such in- 

 gredients is probably small and approximately constant. In 

 practice it is ignored. 



Even when all the necessary precautions are observed, the 

 results obtained in different experiments are not entirely con- 

 cordant, and the coefficients of digestibility are not very 

 reliable. Only those which represent the average of a large 

 number of trials can be accepted. 



The digestibility of the protein in various foods has also 

 been determined by methods of artificial digestion, i.e. by 

 exposing the food successively to the action of pepsin and 

 trypsin under conditions which correspond, as far as possible, 

 to those which obtain in the animal body. The soluble pro- 

 ducts are filtered off, and, after washing, the nitrogen is deter- 

 mined in the residue. Though fairly satisfactory, the results 

 are not considered so reliable as those derived from experiments 

 on living animals. It has not, as yet, been found practicable 

 to determine the digestibility of the fat or carbohydrates by 

 such methods. 



The undigested portion of the food consists, perhaps mainly, 

 of those constituents which, owing to their peculiar physical or 

 chemical condition, resist the action of the ferments ; but it 

 also includes some of the more readily digestible constituents 

 which are so enclosed in the cells of the more resistant material 

 that they are protected from the action of the digestive juices. 



Any imperfection of the teeth or other cause which interferes 



