176 FOODS [CHAP. 



attack of the digestive ferments in its passage through 

 the alimentary tract, and so be in part excreted 

 unchanged. Obviously the test of non-digestibility is 

 the occurrence of the material in the faeces, hence if we 

 wish to determine the digestibility of a given food we 

 must maintain an animal on a diet consisting of the food 

 in question over a particular period. During this period 

 the weight of the food given and the faeces excreted is 

 exactly observed. Both food and faeces are analysed 

 by similar methods; the difference between the fat 

 that has been fed and the fat contained in the faeces 

 gives the proportion of this constituent that has been 

 digested, similarly for the protein and the carbohydrates. 

 When concentrated foods which cannot be fed by them- 

 selves are in question, e.g. linseed cake, a preliminary 

 diet of hay alone is given and the faeces analysed, then 

 for a certain period a known amount of linseed cake is 

 added to the hay diet, and the change in the composition 

 of the faeces is determined. Certain errors are inherent 

 in this process ; in the first place, it is impossible to 

 mark off exactly when the faeces corresponding to a 

 given diet begin and cease to be excreted. Then a 

 certain amount of waste tissue containing nitrogen 

 which had previously been stored in the body is always 

 excreted into the intestine, and so gets reckoned as 

 undigested protein. Again, the bile and other secretions 

 provide matter which is soluble in ether but is not fat. 

 Still the results obtained are close enough to the 

 processes going on in digestion to be of real value. 

 Greater discrepancies are introduced by the fact that the 

 digestive power of one animal differs considerably from 

 that of another, and besides this personal idiosyncrasy 

 one kind of animal possesses greater digestive powers 

 than another, especially for the cellulose and fibre 

 portions of the food. It has already been mentioned 



