CHAPTER VII. 

 DIGESTION. 



THE purpose of the digestion is to separate those constituents 

 of the food which serve as the nutriment of the body from those 

 which constitute the waste, and to separate each in such a form 

 that it may be easily taken up by the blood from the alimentary 

 canal and conveyed to its proper organism. This demands not 

 only mechanical but also chemical action. The first action which 

 essentially depends on the physical properties of the food consists 

 in a tearing, cutting, crushing, or grinding of the food, and serves 

 chiefly to convert the nutritive bodies into a soluble and easily- 

 absorbed form, or in the splitting of the same into simpler combi- 

 nations for use in the animal synthesis. The solution of the 

 nutritive bodies may take place in certain cases by the aid of water 

 alone, but in most cases a chemical metamorphosis or splitting is 

 necessary, and is effected by means of the acids or alkalies, or by 

 the fluids secreted by the glands. The study of the processes of 

 digestion from a chemical standpoint must therefore begin with 

 the digestive fluids, their qualitative and quantitative composition, 

 as well as their action on the nutriments and foods. 



I. The Salivary Glands and the Saliva. 



The salivary glands are partly albuminous- glands (as the parotid 

 in man and mammalia and the submaxillary in rabbits), partly 

 mucous-glands (as some of the small glands in the buccal cavity and 

 the sublingual and submaxillary glands of many animals), and 

 partly mixed glands (as the submaxillary gland in man). The 

 alveoli of the albumin glands contain cells which are rich in 

 albumin, but contain no mucin. The alveoli of thejnucin -glands 



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