DIGESTION 299 



in one great class, the proteins. 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 (water and salts) are never absent. 



Now, although it is by no means necessary that a substance 

 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 proteins, carbo- 

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

 principles have to be obtained from the raw material of the food- 

 stuffs 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 pre- 

 paration is partly mechanical, 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 col- 

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

 diffusible and readily soluble sugars, and the natural proteins 

 into diffusible peptones, and eventually in great part, at least 

 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 molecule into amino-acids and the other groups yielded 

 by its decomposition (p. 332) 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- 



