CHAPTER IX. 

 DIGESTION. 



THE purpose of digestion is to separate those constituents of the 

 food which serve as nutriment for the body from those which are use- 

 less, and to separate each in such a form that it may be taken up by the 

 blood from the alimentary canal and employed for various purposes in 

 the organism. This demands not only mechanical, but also chemical, 

 action. The first action, which is essentially dependent upon the physical 

 properties of the food, consists in a tearing, cutting, crushing, or grinding 

 of the food, while the second serves chiefly in converting the nutritive 

 bodies into a soluble and easily absorbable form, or in splitting them into 

 simpler compounds for use in the animal syntheses. The solution of the 

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

 but in most cases a chemical metamorphosis or cleavage is necessary; 

 this is effected by means of the acid or alkaline fluids secreted by the 

 glands. The study of the processes of digestion from a chemical stand- 

 point 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 mammals and the submaxillary in rabbits), partly mucous 

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

 lingual and sub-maxillary glands of many animals), and partly mixed 

 glands (as the submaxillary gland in man) . The alveoli of the albuminous 

 glands contain cells which are rich in protein but which contain no mucin. 

 The alveoli of the mucin-glands contain cells rich in mucin but poor in 

 protein. Cells arranged in different ways, but rich in proteins, also occur 

 in the submaxillary and sublingual glands. According to the analyses 

 of OIDTMANN 1 the salivary glands of a dog contain 790 p. m. water, 200 

 p. m. organic and 10 p. m. inorganic solids. Among the solids we find 

 mucin, proteins, nucleoproteins, nuclein, enzymes and their zymogens, 

 besides extractive bodies, leucine, purine bases, and mineral substances. 



1 Cited from v. Gorup-Besanez, Lehrbuch d. physiol. Chem., 4. Aufl., 732. The 

 figures there given amount to 1010 parts instead of 1000 parts. 



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