英文科學(xué)讀本 第五冊·Lesson 24 Animal Food

Lesson 24 Animal Food

We learnt from our last lesson on food that the little, growing animal requires three distinct kinds of food-materials—tissue-formers, heat-givers, and bone-makers—if its body is to increase in size and strength, said Mr. Wilson, at the next meeting of the class.

But our lessons of last year's course showed us that food has other work besides that of building up the growing body of the young animal. All animals, whether young or old, are more or less active, and perform many varieties of work with their bodies. Man works not only with his body but also with his mind. How does this work affect the body? Can you tell me, Fred?

Yes, sir, replied Fred. "I think I can explain it. Every act of our daily life destroys some part of the substance of the body. The muscles, as the movers of the body, the brain, as the center of thought and intellect, the eyes, ears, nose, skin—every part of the body destroys some of its own substance in the very act of performing its work. This is why we feel faint and tired after exertion. We have a desire for food. We are hungry. If we were kept without food, our bodies would shrink in size, and we should become weaker and weaker."

First-rate, my lad, said Mr. Wilson, and his eye sparkled with pride at Fred's intelligent answer. "The tissues of the body must be renewed, and built up again, as they are destroyed, or the body would lose in weight and strength. The blood does the work of repairing and making good all losses, as well as that of building up the growing body. It is the food which we take that supplies the blood with the materials for this work.

Our daily diet should contain all three kinds of food-material in some form or other, but not in equal quantities. We require daily just sufficient tissue-forming and bone-making materials to renew the waste which is going on, and just enough fuel to create the necessary amount of heat. It is found that the body needs about four times as much heat-giving, as tissue-forming food, and not more than a quarter of that amount of bone-making or mineral food. If we take more, or less, than these proportions, the body will suffer in some way or other. These food-materials may be obtained, as we have seen, from both the animal and the vegetable world. The flesh itself of certain animals provides our main supply of animal food. The sheep gives mutton and lamb, the ox beef and veal, and the pig pork, bacon, and ham. Poultry (fowls, ducks, geese, turkeys, and pigeons) supply nourishing flesh-food of another kind. The flesh of the deer gives us venison, and certain other wild animals, such as hares, partridges, pheasants, grouse, and woodcock, are called game.

Flesh-food is specially valuable because it is very similar in its nature to the fleshy parts of our own bodies. The muscular or lean, fleshy parts of the meat contain the proteid substance, myosin, the very identical material of which our own bodies are made. It is therefore specially well-fitted for building up our own tissues. Mixed with the lean of the meat, however, there is always more or less fat. The amount varies in different animals, and depends upon their manner of feeding. This fat is, as we have seen, an important heat-giving food. Taken into the body, it does not build up the tissues, but it burns, and in burning creates the heat which the body requires. The value of flesh-food varies in proportion to its digestibility. Mutton, venison, poultry, and game are easily digested; beef, though it contains much nourishment, requires a more robust digestion; pork and veal are less digestible than either.

The quality of the meat, both for flavor and digestibility, may be readily detected by the juices in the lean, which ought to give a delicate color and softness to the flesh. This fact should never be forgotten, when one is choosing a joint of meat at the butcher's. The final value of the meat, both as regards economy, and also its excellence of flavor, depends largely upon the cook. We must not leave this part of our subject without a word or two as to the high value of the bones as food-material. This is a fact by far too little known or appreciated. In the great majority of homes the bones are discarded, and thrown aside as useless and altogether unprofitable. This is a great mistake.

All bone consists largely of ossein—the substance we saw in the bone that had been soaking in the acid. This ossein, when boiled, yields a glue-like substance called gelatine, which is a very valuable tissue-forming food. Leg-of-beef bones make excellent soup, 6 lbs. of the bones being sufficient to yield as much nutritious matter as 1 lb. of the actual flesh itself. What waste it is to throw such bones away! While considering the bodies of animals, we must not pass over fish, for this provides a very valuable article of food, and one which will compare very favorably with butcher's meat. Many kinds of fish are used as food—the commonest are the herring, cod, mackerel, whiting, haddock, plaice, sole, shad, and salmon.

The fleshy part of the substance of all kinds of fish consists of essentially the same materials as those which make up the flesh of other animals, except that it contains more water in proportion, and consequently less nutritive matter. Fish, although not so satisfying as beef or mutton, is very rich in a certain class of mineral matters known as phosphates, and on this account makes an invaluable brain-food. The cheaper varieties of fish contain more nutritive matter than some of the more expensive kinds. Thus, herrings contain a very large amount of flesh-forming material; salmon contain much less in proportion.

Some fish, such as the herring, pilchard, eel, sprat, mackerel, and salmon, contain a large quantity of oily fat just beneath the skin. These are usually known as fat fish. Some, such as the sole, whiting, haddock, plaice, turbot, brill, and cod, have very little fat. They are known as white fish.