PRELIMINARY ANATOMICAL AND CHEMICAL DATA 321 



that upon the whole no diet is sufficient for man unless it contains 

 representatives of all; a proper diet must include proteins, carbo- 

 hydrates, fats, inorganic salts, and water. These proximate prin- 

 ciples have to be obtained from the raw material of the foodstuffs 

 that is, as regards the first three groups, which can alone yield 

 energy in the body, from the tissues and juices of other living things, 

 plants or animals; it is the business of digestion to sift them out and 

 to prepare them for absorption. This preparation is partly mechan- 

 ical, partly chemical. 



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

 are ready for absorption without change. Fats are split into 

 glycerin and fatty acids before absorption. Indiffusible colloidal 

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

 and readily soluble sugars, and the natural proteins into diffusible 

 peptones, and eventually into much simpler decomposition products. 

 These changes are obviously favourable to absorption. But this is 

 not their whole significance. For disaccharides, such as cane-sugar, 

 maltose, or lactose, although easily soluble in the contents of the 

 gut, and in themselves perfectly capable of being absorbed without 

 change, are, unless present in unusually large amount, all converted 

 into monosaccharides, such as dextrose, levulose, or galactose, either 

 in the lumen or in the wall of the alimentary tube. The reason is 

 that the disaccharides are unsuitable as pabulum for the cells. 

 Digestion is not only a preparation of the food for absorption by 

 the gut, but for assimilation by the tissues after absorption. An 

 equally important instance of this double function is seen in the 

 digestion of proteins. The complete shattering of the protein mole- 

 cule into amino-acids and the other groups yielded by its decom- 

 position (p. 360) is required, in the case of that portion of the protein 

 which goes to build up the tissues, because of the high degree of 

 specificity of the tissue proteins. The myosinogen of beef cannot 

 be cobbled into the myosinogen of human muscle, still less we may 

 suppose into the serum-albumin of human blood. It is necessary 

 that the food protein should be completely ' wrecked ' in digestion 

 so that protein which is to take its place in protoplasm may be built 

 exactly to order from the bricks. A satisfactory ' fit ' cannot be 

 obtained with ready-made protein. 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 work 

 of the stomach, but it is normally begun in the mouth, and it is of 

 consequence that this preliminary stage should be properly performed. 



SECTION II. THE MECHANICAL PHENOMENA OF DIGESTION. 



Mastication. -It is among the mammalia that regular mastication 

 of the food first makes its appearance as an important aid to diges- 

 tion. The amphibian bolts its fly, the bird its grain, and the fish 



