"Now that we are clear as to the constituents of which the body is formed, we can proceed to examine the different kinds of food necessary to support the body. Suppose we go back once more to the hen"s egg," said Mr. Wilson. "I will break it, as before, in a cup. Notice the clear, sticky fluid in the cup, and notice too the change that takes place in this fluid when I pour it into boiling water. It changes to a white, opaque, solid substance. We call it white of egg. Its scientific name, albumen, is given to it because of this white appearance; the Latin word albus means white.
"Albumen is a proteid or tissue-former. It was with this as its food that the little chicken built up its tissues while in the egg-shell. Let us next turn our attention to milk, Nature"s other food. I have some new milk in this glass. I will pour a little of this rennet into it, so that you may observe the change that takes place. It curdles at once into a white, opaque, solid substance-the curd. The scientific name for this curd is casein. It is the proteid or tissue- forming constituent of milk. It is with this substance that the young sucking mammal builds up its tissues.
"The food which every animal afterwards seeks for itself must contain proteid or tissue-forming matter of some kind, because its body has to grow, and its tissues can only be formed from these materials.
"Let us now go back once more to the little chick. Its body, you know, felt warm immediately it left the egg- shell; the bodies of all mammals and all birds are warm, and must be kept warm. Do you remember whence the chick obtained the heat which warmed its body?""Yes, sir," replied Fred. "You told us that in the yolk of the egg there is laid up a store of oily, fatty matter. It is this fatty matter which supplies the heat. In like manner, there is in milk an oily, fatty constituent. We can separate it from the milk, and when we have done so, we call it cream. The cream in the milk of mammals serves exactly the same purpose as the oily matter in the yolk of the egg. It supplies the little body with the necessary heat.""Just so," said Mr. Wilson, "and I want you to understand clearly that, as soon as the little creatures begin to seek their own living, they must combine with their other food something which will serve the same purpose, in order that the bodily heat may be kept up. We ourselves, in like manner, require heat-giving as well as tissue-forming food,because it is this which supplies all our vital heat andenergy."
"I remember, sir," said Fred, "you called both the oil of the yolk of egg and the cream of the milk Nature"s fuel-foods and you spoke about these fuel-foods burning. You don"t really mean that they burn in our bodies; do you, sir?""Yes, my lad," replied Mr. Wilson. "I mean that these fuel-foods burn in our bodies, as surely as the coal burns in the grate, although there is no flame, no smoke, such as we usually find when things burn. One of these days we will discuss this matter more fully. You must be contentnow to know, first, that these things and others of a similar nature are called fuel-foods, because, like ordinary fuel, they burn, and secondly, that they actually burn in our bodies, and supply heat and vital energy.
