英文科學(xué)讀本 第五冊(cè)·Lesson 14 Constituents of the Body

Lesson 14 Constituents of the Body

We are going to take another step this morning, began Mr. Wilson, "in our investigations into the subject of food. The little chick and the young mammal of our former lesson showed us clearly that, from the first, the living body requires to be continually built up with food, if it is to grow in size and strength. Now, as the food is the actual material with which this building-work is done, it necessarily follows that it must contain the very essentials which form the substance of the body itself. For example, just as bricks would be altogether out of place for enlarging or repairing a steam-engine built of iron and steel, and no other material but iron and steel would be suitable for the work, so it is in the living body. Its substance can be built up only with the same kind of material as that of which it is itself composed.

Before, therefore, we can discuss the suitability of anything as an article of food, we must learn something of the material of which the body itself is made. Suppose we begin with the flesh. Think of a piece of lean flesh—a joint of beef in the butcher's shop. It looks very solid as it hangs there, but if we were to place it over a fire, we should see it begin to give off vapor, and it would continue to do so until it had lost 77 per cent of its weight. That is to say, a piece of beef weighing 100 lbs. would weigh only 23 lbs. when it had been perfectly dried. We know that the vapor which it gives off takes nothing with it. It is merely water in the gaseous form. Hence we learn that this solid lump of beef weighing 100 lbs. contained no less than 77 lbs. of water.

The 23 lbs. of solid matter left behind are mainly a substance known in the scientific world as myosin. This myosin consists essentially of albumen, the very substance we found in the hen's egg, and it is the chief solid constituent of all flesh. It forms the muscular parts (i.e. the flesh) in our own bodies, as well as in the bodies of other animals. Let us next turn to the bones, and see what we can learn about their composition. I have here a bone that has been lying for the last day or two in this dish of dilute muriatic acid. You will perhaps know the acid better under its more common name—spirits of salt. We will remove the bone from the acid, dry it, and examine it. The first thing to notice is that, while it retains its form, it has lost the hard, firm, rigid nature usual in bone, and is now quite flexible. We may bend it or twist it as we please, and we can cut it easily with a knife. The acid has acted on the bone by dissolving out from it all the material that made it hard and rigid. That which remains is a substance known as ossein. If we boiled this in water, it would yield a sort of glue which we call gelatine. This substance is not only the chief constituent of bone, but also of skin, nails, hair, and feathers.

These two substances, the myosin of the muscular flesh and the ossein of the bone, are the materials which we spoke of in our last lesson, under the name of proteins, or tissue-formers. They are the principal building materials of the body. Both of them were provided in the white of the egg (albumen), and in the milk of the nursing mammal. It is important to remember that the particular protein for building up bone substance will not make muscle or flesh, and vice versa. Now let us look at the structure of bone from another standpoint. Here is a bone, or rather all that is left of a bone, similar to the one I took out of the acid. This has been burnt in the middle of the fire. Take it in your hands and examine it. The first thing you notice is that although, like the other, it retains its original form, it has lost weight; it is very light indeed. Try to bend it, and it at once breaks up in your fingers, for it is very brittle.

The fact is, all the glue-like ossein has been burned out of this bone in the fire, and that which remains is earthy, mineral matter. The gristle-like ossein, which forms certain parts of the body of the animal, is changed into actual bone by these earthy or mineral matters. Lastly, we will turn to the blood. A quantity of blood, dried over the fire in the same way as we dried the flesh, would show a loss of 76 parts of water out of every 100 parts, thus leaving about 24 parts of solid matter. This solid matter consists chiefly of the two substances of which the flesh and the bones are formed—myosin and ossein, together with a varying amount of the earthy or mineral matter.

It must be borne in mind that the blood has to build up and nourish the bony as well as the fleshy structures of the body. This is why we find in the blood all the materials for the work.