7 8 NUTRITION OF FARM ANIMALS 



of a cavity or cavities communicating with the external world 

 by two openings, one for the reception of feed and the other 

 for the rejection of waste. In domestic animals, the digestive 

 tract is large and of very complex structure, but in all cases it 

 is built upon the general plan just outlined. -Always, from the 

 ameba up to man, the inner surface of the digestive cavity is 

 morphologically simply a continuation of the external surface 

 of the body, turned in as one might a glove finger. Conse- 

 quently, the material contained in the digestive cavity, strictly 

 speaking, is still outside the body. 1 



Finally, as an essential part of the digestive apparatus, there 

 must be such organs as the cilia, tentacles, proboscis, lips, etc., 

 by which feed is grasped and introduced into the digestive cavity, 

 and likewise means by which it may be mechanically ground 

 to fit it for the process of digestion, as, for example, the teeth 

 of mammals, the bills and gizzards of birds, etc. 



For the present purpose, it is unnecessary to enter into any 

 elaborate consideration of the anatomy of the digestive organs, 

 since we are concerned chiefly with the chemical rather than 

 the physical processes of digestion, and this section may be 

 confined to a very general description of the digestive organs 

 of domestic animals. In these animals, the digestive apparatus 

 may be described briefly as a tube having various enlargements, 

 folds and diverticula. 



114. Digestive fluids and enzyms. In the ameba, what- 

 ever changes are effected in the substances which it takes as 

 feed are accomplished by the cells of the introverted surface 

 or by their secretions. As the digestive apparatus becomes 

 more complicated, however, a division of cellular labor takes 

 place and certain groups of cells are set apart to produce the 

 digestive juices which act upon the feed. In the higher animals, 

 these cells become the numerous secreting glands which are 

 an essential part of the organs of digestion. The principal 

 active agents in digestion are certain enzyms secreted by these 

 glands, the more important digestive enzyms in the higher 

 animals being : 



i. The amylases, ptyalin (in 'the saliva) and amylopsin (in 

 the pancreatic juice), acting upon starch. 



1 For a more complete discussion of the development of the digestive apparatus 

 see R. Meade Smith, The Physiology of the Domestic Animals, pp. 203-226. 



