3. Colorless objects called white corpuscles. Of these there are only about six hundred thousand to each drop of blood, although the number varies greatly from time to time. They are specks of jellylike substance that change their shape constantly. They not only travel with the other corpuscles in the plasma, but they also work their way through the walls of the capillaries and wander here and there in the body. They destroy intruding microbes when they find them, and help more than any other part of the blood in healing a wound. Much more is told about these white corpuscles in the last two chapters of this book.

Plasma, red corpuscles, and white corpuscles tell us all that the microscope shows when we use it for the study of blood. But a chemist will take the same blood, will analyze it in his laboratory, and will prove that it is made up of many different substances of which we have not so much as heard the names, - substances needed, however, for the work which each separate part of the body is doing. He will tell us that within this blood is all that is needed for the manufacture of bone and muscle, hair and tendon, tears and fat and finger nails; that it is the source of supply for all that lies under the cover of the skin, the storehouse for more treasures than we have even dreamed about; and that it is easy to enrich or to impoverish the blood by our treatment of the body.

CHAPTER XIII

EXCHANGES ALONG THE TUBES

Even a careless thinker will see that however intricate the lacework of capillaries is, and however closely these small tubes are intertwined with

tissues of muscle and gland, still the blood within the tubes is useless to the body unless it can be brought into direct contact with the muscle and gland tissues themselves.

An experiment will make the situation plain and will show what the outcome of it is.

ONE GLASS WITHIN THE
OTHER

The smaller glass holds fresh
water, the larger holds water
and salt

Get from the butcher a piece of fresh animal membrane, the bladder will do. Fill a small glass with fresh water, tie the membrane tightly over it, set the glass into a much larger one filled with salted water, letting the water cover it, and leave the two tumblers together over night. In the morning take the smaller from the larger, unfasten the membrane, and taste the water which was fresh and sweet the night before. You will find that it is now distinctly salt. Taste

the water in the larger tumbler. You will find that it has grown fresher than when you left it.

In this exchange the salt in the liquid has acted according to a universal law. Salt is indeed one of the many substances which always pass easily back and forth through any moist animal membrane.

Put sugar into one liquid and soda into another; let a membrane be stretched between them, and before long you will have two liquids that have become strangely alike. The different substances in the liquids have changed places through the membrane.

Even gases are subject to the same law. Men who know how to handle such things can put oxygen in one tube and carbon dioxid in another. They can then arrange to separate the gases by a piece of animal membrane stretched between the tubes, and they discover that the two gases refuse to stay apart. Indeed, so much of each finds its way through the partition that soon there is a mixture of the two on either side of the membrane.

Experiments such as these answer the query as to how the body gets what it needs from the blood. Everywhere it is the animal membrane of the tubes themselves which separates the blood within the tubes from a certain other liquid which lies close about them on the outside.

However small and however thin walled the blood vessels may be, there is always this lymph bathing the

outside like a sort of colorless sap in the body, and making its exchanges with the contents of the liquid within the capillaries. Moreover, this lymph which soaks slowly but constantly through every tissue of the body is laden. with carbon dioxid which it has received from the tissues of the body. The blood is rich in oxygen, and it is separated from the lymph only by the walls of the capillaries. In view of this, what could be more natural than the thing which comes to pass? These gases in the lymph and in the blood change places with each other as promptly as do the liquid materials which are also in the lymph and in the blood.

It is evident, then, that the lymph is as important to us as is the blood itself. In fact, the two must always travel side by side. They are indispensable to each other. Without the one the other is useless. Three statements will show how close the relation is:

1. Blood in the arteries is the result of the food we eat and of the air we breathe. It contains every supply that any part of the body needs for nourishment, for strength, and for growth.

2. Blood in the veins is what is left after the lymph has taken from it the oxygen and other nourishment which the body needs, and given in exchange the carbon dioxid and other waste which must be carried off. In other words, venous blood is rich in waste from the tissues and poor in nourishment for the tissues.

3. Lymph is made up of rich, nourishing plasma from the blood, on its way to the tissues, and of waste material from the body, which will soon pass into the capillaries, be carried onward in the veins, and be disposed of as we shall learn hereafter. Lymph is also the highroad to the blood for many substances that are being manufactured by the different organs of the body. These manufactured articles must find their way into the blood, for only through circulation will they ever be able to reach their destination.

The origin of the lymphatic tubes is strangely interesting for the simple reason that it is so very indefinite. Each seems to begin about as irregularly as a stream gathers water in a swamp.

As we know, blood vessels are a closed system of tubes with a stream of blood sweeping through them endlessly, -going ever round and round, from heart back to heart again. In this great system not even the smallest tube in the remotest region of the body is left with an open mouth. The lymphatic system, however, works on quite a different basis. Here the vast multitudes of the smallest tubes seem to be really little more than open mouths into which liquid is gradually making its way. Bear this in mind while the facts are given as definite statements:

1. Each blood vessel of the body makes its way through a mesh work of tissues.