The science of life : fully illustrated in tone and line and including many diagrams

THE GOMPLEX BODY-MACHINE AND HOW IT WORKS

phosphate of lime, a very insoluble compound, which would form clogging masses of crystals in the delicate tubules of the kidney if it were excreted by this organ. This substance is excreted into the large intestine from the blood-vessels in the intestinal wall and ejected from the body with the faeces. Other mineral substances are also removed in this region.

8 7 How our Food becomes Blood

We have now made a very extensive examination of Mr. Everyman’s internal workings. Since we chose him (with occasional allusions to his attendant mouse) for our first examination of a living thing, and noted his extraordinary pre-occupation with feeding and his close parallelism to a machine, we have illuminated his structure and the nature of his mechanism very considerably. We now know him and _ the mouse his parasite to be extraordinarily complex organizations of billions of inferior beings, his cells; we know that within his skin all his body is soaked in blood and lymph ; we have magnified his structure, made him, so to speak, transparent, dragged his lungs and heart and kidneys into the light, and exhibited them in illustrations, and shown how part works with part while he (almost regardless of his immense multitude of detailed activities within) goes through the routines of his daily life. But there still remains a gap before our opening review of the routine working this human bodymachine is complete. We know that he needs food to supply the blood with the fuel to keep his billions of constituent cell machines going, but we have still to trace how the food he packs so sedulously into himself becomes the warm, nutritious, comforting blood which bathes all his internal being. To that we will next address ourselves.

It is possible now to draw up a rough list of the different kinds of food that he requires. We know that he needs fuel—matter which is capable of yielding, when oxidized, the supply of energy without which life cannot continue. The substances of this class are not built into his living structure, but are taken up and stored in his cells, to be burnt whenever required ; they correspond to the petrol that runs through a motor, rather than to the machine itself. Besides fuel, he needs materials for growth and for making up for the slight wearing away of the living engine that any activity involves. Then

there are substances, such as the water or salt of blood, that are neither built up into protoplasm nor used as fuel, but are nevertheless necessary constituents of the bathing fluid of cells. Lastly, we add to this list certain “accessory food factors,’ the socalled vitamins, whose réle is still obscure, and which we shall discuss more fully in a later book.

Life, however, is not so simple that he can sit down to his meals and take so much fuel, so much tissue material, so much water, and so much of the minor helps and stimulants. Certain types of restaurant indeed mask the dishes with rather doubtful indications that they are flesh-formers or energy-generators. But life is not so straightforward. Mr. Everyman has to take his food as he finds it, and it has not undergone nearly as much sorting out as that. The simplest item in his consumption is the water he drinks, either pure or with various colouring, flavouring, or stimulating additions. Of that, therefore, we will speak first. We have already stressed the profoundly important part that water plays in the living machine. About 59 per cent. by weight of the human body is water, the proportion varying from tissue to tissue. Thus bones, which include a large amount of lime between the cells, have only 22 per cent. ; but the liver contains 69 per cent., muscle 75 per cent., and the kidney, which is comparable to a sponge of tubules whose pores are full of blood and urine, contains as much as 82 per cent. of water. The inside of cells is fluid and consists largely of a solution of various substances in water. Moreover, water acts in certain characteristic ways on molecules which are dissolved in it, so that the properties of aqueous solutions —upon which vital phenomena dependare unparalleled by other mechanical systems. Life without water is about as conceivable as music in a vacuum. As Sir Arthur Shipley says in his attractive little book Life, “* Even the Archbishop of Canterbury comprises 59 per cent. of water.” Water is continually being lost by the body. It evaporates from skin and lungs, and it is used also to flush away the waste products of cell activity. To make up for this loss, a resting man needs somewhere about three pints of water a day; a man taking considerable physical exercise much more.

In addition, Mr. Everyman must swallow a variety of other substances, chiefly the more or less altered living or dead tissues of plants and other animals, and trust his intricate internal arrangements to assimilate them to

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