THE ALIMENTARY CANAL 183 



Classification of Foods. In Chapter II the contrast be- 

 tween proteins and non- protein organic foods is briefly 

 indicated. The former contain carbon,. oxygen, nitrogen, 

 hydrogen, and sulphur, sometimes phosphorus. The 

 latter are non-nitrogenous. They may be grouped as 

 below: 



Carbohydrates, including starches and sugars. 



Fats (or oils). 



Alcohol. 



Starches are incompletely soluble, of large molecule, 

 and tasteless. They are easily converted to sugars 

 which are freely soluble, of small molecule, and sweet. 

 Fats have familiar physical characters: low melting- 

 points and insolubility in water. They contain the same 

 elements as carbohydrates: carbon, hydrogen, and oxy- 

 gen (much more of the first, much less of the last). 

 Water and mineral salts are reckoned as inorganic foods. 

 The Nature of the Digestive Changes. Digestion is 

 often said to be a preparation of food for absorption and 

 this is correct if it is understood in the broadest sense. 

 A narrow conception is to be avoided. First of all, 

 we must not think of digestion as mere solution. Before 

 the rise of organic chemistry it was scarcely possible to 

 have any other idea concerning it and observers judged 

 its progress simply by the dissolving of food samples. 

 It is true that solid food must be dissolved but there 

 are other aspects of the process to be taken into account. 

 Foods which are in solution may yet require to undergo 

 digestion. 



Digestion is a process of refining. It effects a separa- 

 tion between the valuable and the useless. But, again, 

 this is only one feature of {he change. When a food 

 already soluble is digested it is said to gain in diffusi- 

 bility. By this we mean that the power to pass through 

 ordinary membranes, like parchment, is increased. 

 This may be supposed to make absorption easier but, 

 once more, the gain in diffusibility is but one of several 

 phases in the transformation which we call digestion. 



