88 THE FEEDING OF ANIMALS 



to which the nutritive substances are conveyed by the 

 blood. The two terms are entirely distinct in meaning, 

 although they are confused in popular speech. 



120. General changes in food through digestion. 

 In digestion, food undergoes both mechanical and chemi- 

 cal changes. It is masticated, that is, ground into 

 finer particles, after which, in its passage along the 

 alimentary canal, it comes in contact with several 

 juices which profoundly modify it chemically. That 

 portion of it which is rendered diffusible is absorbed 

 by certain vessels that are embedded in the walls of 

 the stomach and intestines, and is conveyed into the 

 blood. The insoluble part passes on and is rejected by 

 the animal as worthless material, and constitutes part 

 of the solid excrement or feces. The forms in which the 

 nutrients are conveyed into the circulation are believed 

 to be the following: The proteins, previous to absorption 

 into the blood, are converted into soluble bodies, at 

 first proteoses and peptones, and finally into simpler 

 nitrogen compounds (amino acids) resulting from a more 

 extensive cleavage; the carbohydrates enter the blood 

 as sugars, chiefly as dextrose. The fats are changed into 

 a finely divided form, either as such or as fatty acids and 

 soaps. A study of digestion includes, then, a knowledge 

 of mastication, of the sources, nature, and functions of 

 the several digestive juices, and a consideration of the 

 various conditions affecting the extent and rapidity of 

 digestive action. 



FERMENTS 



The changes involved in rendering food compounds 

 soluble are intimately connected with a class of bodies 



