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CHAPTER VIII.
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 alone,
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 submaxillary 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 Magnus-Levy 1
the human salivary glands contain 274 p. m. solids,
of which 114 p. m. was fat and 154 p. m. was protein. Among the
solids we find mucin, proteins, nucleoproteins, nuclein, enzymes and theix
1
Bioch. Zeitschr., 24.
451
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