282 A MANUAL OF PHYSIOLOGY 



pharynx and oesophagus, the glands lie in the submucosa, as do the 

 glands of Brunner in the duodenum ; everywhere else they are con- 

 fined to the mucous membrane proper. Between the openings of the 

 glands the mucous membrane is lined with a single layer of columnar 

 epithelial cells, sometimes (in the small intestine) arranged along the 

 sides of tiny projections or villi. At the ends of the alimentary canal, 

 viz., in the mouth, pharynx and oesophagus, and at the anus, the 

 epithelium is stratified squamous, and not columnar. 



The purpose of food is to supply the waste of the tissues 

 and to maintain the normal composition of the body. In 

 the body we find a multitude of substances marked off from 

 each other, some by the sharpest chemical differences, others 

 by characters much less distinct, but falling upon the whole 

 into a few fairly definite groups. Thus, there are bodies like 

 serum-albumin, serum-globulin, myosin, and so on, which 

 are so much alike that they can all be placed in one great 

 class, as proteids. Then we have substances like glycogen 

 and dextrose, vastly simpler in their composition, and 

 belonging to the group of carbo-hydrates. Then, again, fats 

 of various kinds are widely distributed in normal animal 

 bodies ; and inorganic materials, such as water and salts, are 

 never absent. 



Now, although it is by no means necessary that a sub- 

 stance in the body belonging to one of these great groups 

 should be derived from a substance of the same group in the 

 food, it has been found that no diet is sufficient for man 

 unless it contains representatives of all ; a proper diet must 

 include proteids, carbo-hydrates, fats, inorganic salts and 

 water. These proximate principles have to be obtained 

 from the raw material of the food-stuffs ; it is the business 

 of digestion to sift them out and to prepare them for 

 absorption. This preparation is partly mechanical, partly 

 chemical. 



The water and salts and some carbo-hydrates, such as 

 dextrose, are ready for absorption without change. Fats are, 

 probably, for the most part, only mechanically altered. In- 

 diffusible carbo-hydrates, like starch and dextrin, are changed 

 into diffusible sugar, and the natural proteids into diffusible 

 peptones. Mechanical division of the food is an important 

 aid to the chemical action of the digestive juices. We shall 

 see that this mechanical division forms a great part of the 



