[Illustration: FIG. 17.--_The Stomach laid open behind._ _a_, the œsophagus or gullet; _b_, one end of the stomach; _d_, the other end joining the intestine; _e_, gall duct; _f_, the gall-bladder; _g_, the pancreatic duct; _h_, _i_, the small intestine. ] We eat all manner of dishes, but in all of them that are worth eating we find the same kind of things, which we call =food-stuffs=. We eat various kinds of meat; but all meats are made up chiefly of two things: the substance of the muscular fibre, which you have already learnt is a =proteid= matter containing nitrogen, and the =fat= which wraps round the lean muscular flesh. Now, proteids are, when cooked, insoluble in water (see p. 49); and fat, you know, will not mingle with water. Both these parts of meat, both these food-stuffs, must be acted upon before they can pass from the inside of the alimentary canal, through the epithelium of the mucous membrane, into the blood capillaries. Besides meat we eat bread. Bread is chiefly composed of =starch=; but besides starch we find in it a substance containing nitrogen, exceedingly like the proteid matter of muscle or of blood. Potatoes contain a very great deal of starch with a very small quantity of proteid matter; and nearly all the vegetables we eat contain starch, with more or less proteid matter. Then we generally eat more or less sugar, either as such or in the form of sweet fruits. We also take salt with our meals, and in almost everything we eat, animal or vegetable, meat, bread, potatoes or fruit, we swallow a quantity of mineral substances, that is, various kinds of salts, such as potash, lime, magnesia, iron, with sulphuric, hydrochloric, phosphoric, and other acids. =In everything on which we live we find one or more of the following food-stuffs:--Proteid matter, starch or sugar, and fat, together with certain minerals and water. It is on these we live: any article which contains either proteid matter, or starch, or fat, is useful for food. Any article which contains none of them is useless for food, unless it be for the sake of the minerals or water it holds.= We are not obliged to eat all these food-stuffs. Proteid matter we must have always. It is the only food-stuff which contains nitrogen. It is the only substance which can renew the nitrogenous proteid matter of the blood and so the nitrogenous proteid matter of the body. We might indeed manage to live on proteid matter alone, for it contains not only nitrogen but also carbon and hydrogen, and out of it, with the help of a few minerals, we might renew the whole blood and build up any and every part of the body. But, as you will learn hereafter, it would be uneconomical and unwise to do so. Starch, sugar, and fats, contain carbon and hydrogen without nitrogen; and hence, if we are to live on these we must add some proteid matter to them.