英文科學(xué)讀本 第六冊(cè)·Lesson 03 The Blood and its Work

Lesson 03 The Blood and its Work

We have learned, in the course of our lessons, a great many things, in a simple way, about our body, both in regard to its structure and the work it has to do. We shall now enter into a more detailed investigation of the functions of the principal vital organs. The body is a sort of living machine. Some part or other of it is always at work, night and day, sleeping and waking; for even when we are asleep the heart and lungs cannot rest—their work must go on, and that work must be guided and controlled by some part of the nervous system. We know too, from our earlier lessons, that all bodily work is done at the expense of the substance of the body itself. Every act of our daily life, the movement even of a finger, the flashing of a single thought through the brain, or the blinking of an eyelid, destroys some of the substance of the body. Think of a steam-engine at work. It is the fuel in its furnace and the water in its boiler that enable it to perform its work, but the living machine works at the expense of its own substance. Each little particle of brain, nerve, muscle, or skin, moreover, having once performed a certain work, becomes henceforth worn-out, dead matter. Our body lives by dying—some part of the body is dying every moment. Notwithstanding this constant destruction of the substance of the body, a person in good health varies very little in size and weight for years. The reason for this is that all over the body there is not only constant waste, but constant renewal. Bit by bit, every part of the body—muscle, brain, bone—is not only being destroyed by work done, but, as it is destroyed, it is renewed or built up again in its original form. This building-up work is done by the blood. If I prick myself in any part of my body, blood will flow. There is blood in all parts of the body, and it is this blood which builds up what has been worn out. It carries all the materials for building the various tissues. To the muscles it gives certain particular materials for making muscle; to the bones it gives up other materials for making bone; in the brain and nerves it leaves other materials again for making nerve matter, and so on. But it never leaves bone-making materials for building up muscle, brain, or nerve; nor does it take to the ear what is wanted for the eye. It never makes mistakes. We know that there is blood all over the body. But this blood does not stand in the body, as water stands in some vessel; nor is it still and stagnant. It is a restless stream, incessantly on the move, bringing to every part the materials required for building purposes, and carrying away whatever is no longer wanted there. As long as life lasts this stream of the blood must flow on. We say that the blood circulates. We mean to say by this that it does not merely pass through the body like a stream and disappear, but is carried round and round, over and over again, very much in the same way as the hot water circulates through the pipes provided for it in a large building. This thought leads us to the fact that the blood itself circulates through a system of pipes. These pipes commence at the heart, and form a complete circuit through the body and back to the heart again, penetrating into every part except the nails, the hair, and the enamel coating of the teeth. Our hair and nails do not bleed when they are cut. These pipes have the one common name—blood-vessels, although they are not alike, either in structure or the work they have to do. The heart is the center of the circulation. Those blood-vessels through which blood flows from the heart to other parts of the body are called arteries. Those which bring blood back again to the heart are veins. Both arteries and veins branch out continually into smaller and smaller vessels the farther they get from the heart. Between the smallest branches of the arteries and veins are multitudes of extremely fine tubes, like the most delicate hollow hairs. These are called capillaries. The word capillary, as you know, comes from a word that means a hair. The constant stream of the blood, then, flows from the heart, along the arteries, into every part of the body. The arteries are continually splitting up into smaller and smaller branching tubes, until at last they become the merest hair-like vessels, the capillaries. The blood continues to flow through these capillaries, and is collected up, as it flows, by the smallest branches of the veins. The veins gradually unite into larger and larger pipes, as they get nearer the heart, and at last discharge the blood which they carry into the heart itself. From what we have already said about the heart, you will be prepared to learn that it is a hollow vessel. You have all, I suppose, seen a sheep's heart. It will be sufficient for us now to remember that the heart does the work of a pump. It is constantly at work pumping blood into the arteries. From the first dawn of life till death takes us, the heart never rests for a single minute from its pumping. Lesson 04 Plants Useful for Food (Ⅰ) In no part of our daily wants do we show our dependence on the vegetable kingdom so much as in our food. All our food is, directly or indirectly, of vegetable origin; although we eat the flesh of certain animals, the animals themselves derived their support, all through their lives, from plants of one kind or another. Man, in every part of the globe, makes bread of some kind his staple food and, although the bread is not always made of the same sort of material, it is in every case a vegetable substance. The cereals, or corn grains, form the bread-making materials for the greater part of civilized mankind. Indeed, they provide the staple food of more than four-fifths of the population of the globe. These grains contain two important food materials—starch and gluten. One, you already know, is valuable as a fuel-food, the other as a tissue-forming food. The various corn grains owe their relative importance as food-stuffs chiefly to the amount of gluten which they contain. The gluten of the grain always resides in the outer part, immediately under the skin or husk. This explains why the bran of English wheat, which consists largely of this outer skin, contains more gluten than the finest white flour. Fine white flour is obtained only by sifting the meal through sieves after it is ground. The sifting removes all the particles of bran. Sometimes the meal is used without sifting, just as it leaves the mill, in fact. It is then known as whole-meal. The bran of English wheat contains 16 percent gluten; the whole-meal 12 percent; fine white flour rarely contains 10 percent. Bread, therefore, made from whole-meal is more nutritious than that made from fine, sifted, white flour. Reference has been made to the relative value of the corn grains, and it will be well now to compare them one with another. Oatmeal is very rich in gluten; it contains about 16 percent of this substance—that is, about the same proportion as we find in the bran of English wheat. Barley and rye contain about the same proportion of gluten as is met with in wheaten flour, but the meal of both is coarser in flavor and color, and the bread made from them, instead of being light and spongy, is heavy and close. Rye forms the chief food of the masses in Russia. In this country it is grown, and cut green for feeding horses. Rice is remarkable among the corn grains for containing the smallest proportion of gluten. Even in the undressed rice, or paddy, such as is eaten by the people of the East, there is not more than 7 or 8 percent—about half the amount we find in oatmeal, and the grain as we use it contains much less, because, with the removal of the outer skin, much of the gluten is also removed. Maize contains only about 9 percent gluten, but it is richer in fatty or oily matter than any of the other grains. This makes it more easy of digestion, and it is well adapted for fattening animals. Let us see now how Nature suits her foods to the requirements of the various populations of the world. Rice forms the food of fully one-third of the entire human race. The majority of these people inhabit the tropical and sub-tropical regions of the world. From the nature of the climate, they are unable to undergo the physical and mental labors which form the everyday life of man in the more temperate climates; hence the bodily wear and tear are less, and the need of nutrition also less. Rice, with its small proportion of gluten, contains sufficient nutriment for such people. The nature of the hot climate, moreover, has a relaxing effect on the system; rice, containing as it does very little fat, is less relaxing than any other of the corn grains—indeed, it is rather binding than relaxing in its effect. It is therefore just the food suitable for such people and such climates. How remarkable and wonderful then it seems that Nature should supply this special food, so peculiarly adapted to the needs of the people, in those very localities where the other corn grains could not be grown. Notwithstanding all this, Europeans in those countries have frequently been astonished at the enormous quantities of rice which the natives are obliged to devour at a meal, because of the small amount of the necessary gluten which the grain contains. In cold, bracing climates, such as that of Scotland, oatmeal forms the staple article of diet. But this same food would not be found suitable in warmer and more relaxing climates, because oatmeal itself has a decidedly relaxing effect. Here again we see that the special food which Nature provides is a hardy plant, able to withstand the rigor of the climate in which wheat cannot be grown. Owing to the density of the British population, it would be quite impossible for the country to provide sufficient homegrown corn. They therefore import largely from other countries. England's total imports of wheat amount to upwards of sixty-five million cwt. (hundredweights) annually. The United States are England's chief granary, but Russia, India, Canada, South Australia, and Cape Colony all contribute. Maize is imported to the extent of about two million tons every year. It is a most useful grain, but it will not ripen in England's climate. It is mostly used as corn flour, Oswego, and maizena—preparations for making blancmanges, puddings, and biscuits. Of late years, maize has been in great demand for the preparation of glucose, a kind of sugar used in brewing and confectionary. England imports annually from six to ten million cwt. of barley, mostly from the northern countries of Europe. It is chiefly used in brewing, and for feeding cattle and poultry. About three million acres in Great Britain are devoted to the cultivation of oats, and in addition to this they import largely. Buckwheat is extensively grown in America, and makes very nutritious flour. In the backwoods of America it is made into delicious cakes, which are eaten hot with maple-honey. England imports buckwheat for feeding poultry. Dari, or dhoora, is a small kind of grain, which is largely cultivated by the people of India, Egypt, and Central Africa for their own use. It is commonly known as millet, and yields a beautiful white meal, containing from 8 to 9 percent gluten. Lesson 05 Machines We closed our lesson on the Forces of Nature," said Mr. Wilson, "by glancing at the windmill and the water-mill—two contrivances by means of which man is enabled to utilize for his own benefit the natural forces, wind and running water.

Without attempting to inquire into the precise manner in which these mills accomplish their work, we were content to learn through them a new name—machine. We learned that any contrivance for transmitting a force from one point to another, or for altering the direction of movement, is a machine. Let us now take another example. A workman wants to raise a basket of bricks or a pail of mortar from the ground to the scaffold at the top of the building where he is working. He does not wish to descend and remount the ladder, so he lowers a rope. One end of this is hooked on to the basket, and he raises the weight by his own muscular force. This force is actually applied at the upper end of the rope, but the rope transmits the force to the basket. Hence the rope is a machine. It changes the point of application from the upper end of the rope to the basket at the bottom.

It is important to remember that the man raises the basket in this way by exerting a direct upward force, greater than the force of gravity caused by its weight, which is tending to pull it downwards. We will now proceed with a little experiment. I have here a simple contrivance, consisting of a small wheel with a hollow, grooved circumference, the wheel itself being fixed to a bracket. I will pass this cord round the wheel, and to each end of the cord attach a pound weight. The weights, when left to themselves, balance each other; there is a downward force of one pound on either side of the wheel.

Now I will hang a small weight (this half-ounce weight will do) on one side. That, and the pound weight together, immediately overcome the weight on the other side; they fall and it rises. We will now return to the workman's scaffold, and imagine such a wheel set up there. Two baskets of bricks, both pulling downwards by the force of gravity, would balance each other; but one brick more in either basket would cause that one to sink and the other to rise. Suppose the man were to unhook one of the baskets. How could he manage to keep the other in its original position?"

To do this he would have to pull downwards, just as the basket did, and with the same force, sir, replied Fred.

Exactly, and if he pulled a little harder, what would happen then?

I suppose the basket on the other side would rise, sir.

Yes, Fred, it would; that is to say, the man by pulling the cord downwards over the wheel, exerts an upward force on the basket at its opposite end, and causes it to rise. In other words, the wheel and the cord constitute a machine, for they change the point of application, and they alter the direction of the force. Workmen usually employ this means of raising their materials to the scaffold, because it is easier to pull downwards than it is to lift the entire weight upwards.

It frequently happens, however, that the men have to raise heavy masses of material (such as great sheets of lead, or coils of leaden pipe) which it is impossible for them to lift from the ground with only their own muscular force. We will see how they do it. Let us return to our wheel and cord experiment. I will take one part of the cord, as it hangs over the fixed wheel, pass it round and under a similar loose wheel, and then fasten the end of it to the beam above. Here we have a contrivance consisting of a fixed and a movable wheel, with the same cord passing round both. When I pull at the other extremity of the cord, the second wheel actually moves up as the cord is pulled down. Now, we will hook a 2 lbs. weight to the movable wheel, and a 1 lb. weight to the free end of the cord. When left to themselves the two weights balance each other. It would be just the same if we used 4 lbs. and 2 lbs., 100 lbs. and 50 lbs. They would balance, and the slightest additional weight hung on the end of the cord would cause the movable wheel with its heavier load to rise.

This is the kind of contrivance used for raising heavy weights. With such a contrivance fixed to the scaffold, men are enabled, by pulling downwards at the end of a cord, to raise a heavy mass of material, which they could not possibly lift without help. Such a contrivance is a machine. The men apply the force at one end of the rope; it is transmitted to the other. The force is applied downwards; the heavy mass is raised upwards. The small force exerted by the men is increased in magnitude or intensity, so that it is enabled to raise a much heavier weight than it could without such a machine. We employ a vast number of machines to do an almost endless variety of work. Some of them are very complicated. But however complicated they may appear to be, they are all found on examination to be made up of a few simple contrivances, which have been invented for the purpose of altering force in one or more of the ways we have mentioned."