Digestion — Ferments 99 



mentary canal, a passage that begins with the mouth, 

 includes the stomach and intestines, and ends with the 

 anus. Assimilation signifies the appropriation of nu- 

 trients, after digestion, to the maintenance of energy 

 and to the building of flesh and bones, processes taking 

 place in the tissues, to which the nutritive substances 

 are conveyed by the blood. The two terms are entirely 

 distinct in meaning, although they are confused in popu- 

 lar speech. 



In digestion, a feeding stuff undergoes both mechani- 

 cal and chemical 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 ab- 

 sorbed by certain vessels that are imbedded 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 the 

 solid excrement or feces. A study of digestion includes, 

 then, a knowledge of mastication, of the sources, nature 

 and functions of the several digestive juices, and a con- 

 sideration 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 

 known as ferments, to which brief reference has already 

 been made in their relations to the preservation of feed- 



