WE have learned to think of the cells which make up the body as dependent on the fluid which surrounds them for the various materials they require, and as a place into which they discharge the products of their metabolism. We have seen furthermore that the fluids which bathe the cells directly must be constantly renewed. The renewal is accomplished by interchanges between this fluid and the blood, which constantly flows through the tiny blood vessels that are everywhere present in the body. In its course, in turn, it passes through the blood vessels of the organs in which it is to be itself renewed; the digestive organs for food supplies, the lungs for oxygen, the kidneys for the discharge of waste material. We must now look further into the nature and action of the various body fluids. Of course the foundation of all of them is water. In this water must be dissolved everything that is used by any of the cells for food or anything that any of these cells produces. Under this latter head we have the waste products of ordinary metabolism, or in the case of some cells special products of functional metabolism. The presence of all these various materials would be bound to make the body fluid an extremely complex mixture. In addition to these various materials there are certain substances present besides water to make up what we may call the structure of the body fluids as distinguished from the materials which they are carrying from one place to another. These structural materials include a number of salts of which ordinary table salt (sodium chloride) is the most abundant as well as the most familiar. In addition there are salts of lime and potash and magnesium; all these latter in very small proportions. Just why the body fluids should contain these salts is not very clear. We know that if they were not present the cells of our bodies could not live, yet it is true that there are a great many kinds of living cells that get along perfectly well in fresh water, which may have no salts dissolved in it at all. On the other hand there are more kinds living in the ocean and exposed to the rather strong concentration of salts which make up ocean water, and there are even some kinds of animals that live in strong brine, so that evidently living protoplasm can adjust itself to surroundings in which the strength of the salt in solution is widely different. It is true that very few kinds of animals can endure being changed suddenly from ocean water to fresh water or the reverse. One of the best ways to clean the bottom of a ship that has become foul through long sailing about the sea is to transfer it into a fresh water lake or stream, where the accumulation of living animals and plants will be killed and will drop off. Most kinds of ocean animals die rather promptly if changed to fresh water, or fresh water animals if put into the ocean. There are a few kinds of fish, like the salmon and shad, which live in the ocean but lay their eggs in rivers, and these are able to endure the change from the one kind of water to the other without being destroyed. They, to be sure, make the transition gradually, swimming up from the ocean into water that is less and less salty, until they finally reach the fresh water stream itself. There are other kinds of living things which can endure a much more abrupt change from salt to fresh water and back again. In various parts of the world are large rivers emptying into the ocean and so situated relative to towns and cities that steamers make regular round trips from the ocean up to the fresh water of the river and back again, and it will be found that on the bottoms of these steamers are various plants and water animals which endure the frequent shift from salt water to fresh and back again without harm.

The percentage of salt in the body fluids of all the higher animals including ourselves is only about one-fifth that in the ocean. Furthermore the fluids of most kinds of land animals have about the same percentage of salts dissolved in them. Naturally there has been much speculation as to why there should be this percentage of salt in preference to any other. One ingenious theory supposes that back in the beginning of things, when the earth cooled down below the boiling point of water, so that it was possible for water to collect on the earth, the water in all the oceans was fresh. This would have to be true, since water must have fallen in the form of rain; but in course of time some of the salts in the earth’s crust would be dissolved, making the water salty, and as time went on the ocean would become saltier and saltier. This is still happening, for every river that discharges into the ocean carries with it materials that it has dissolved from the underlying soil during its passage from its source, and such material, when it once enters the ocean, must stay there until the ocean water becomes saturated with that particular substance. According to this theory there was a time, then, when the ocean water was just about as salty as our body fluids at the present time, and it supposes also that that was the time when the ancestors of present land animals crawled out of the ocean and took up their abode on land. Of course there is no way to prove that this is so, but it does account for the particular percentage of salts in our bodies as well as any other explanation we know anything about.

In addition to these various salts our body fluids contain in solution moderate amounts of very complex chemical substances belonging to the class of proteins. A fact about proteins which has not yet been emphasized is that they make liquids in which they are dissolved sticky or gelatinous. An excellent example of this is ordinary raw white of egg, which is a solution of protein in water, and which shows the gelatinous character very strikingly. Because of the protein that is in solution in the body fluids, they have also this gelatinous character, although to a much less extent than in the white of egg, because the amount of protein in solution is so much less. As we shall show later, this sticky quality of the fluid is of a good deal of importance in its actual use in the body. It may be that the protein in the body fluids serves other purposes as well. One interesting fact that needs to be emphasized is that it is not used for fuel or building material for the cells. The protein that comes to them as part of their food supply is entirely distinct from that about which we are now talking, which is part of the permanent structure of the body fluids. We shall speak of the protein that serves to nourish the cells in a moment, when we are talking about the relations of the fluids to the transportation of food material.

In addition to the salts and proteins we have also dissolved a great many very complex materials which may be looked upon as permanent or relatively permanent constituents of the fluid, but about which we know practically nothing chemically. We are sure that they are present, because of certain effects which they produce, but the substances themselves have never been made out by chemical analysis. These are materials which are concerned with the resistance of the body to infectious disease, and it will be necessary to say just a word about infection to make clear the part played by them.

What we call an infection is the invasion of the body from the outside by minute living organisms, either plant or animal, and the establishing of them within the body, so that they grow and multiply. They carry on their metabolism just as do all other living cells and produce various chemical products as a result. There are many organisms living within our bodies whose metabolic products apparently do us no harm, and so we serve as hosts for these unbidden guests year in and year out without even knowing of their existence. The products from other kinds of organisms are poisonous to us, and when some of these organisms multiply within us we discover it, because we are poisoned and become ill. Only organisms whose products of metabolism are poisonous are counted ordinarily as causing infection. In the strict sense we might be said to be infected by the harmless organisms of which so many thrive within us, but in the usual use of the word we speak of one as having an infection only when his body has been invaded by organisms that produce poison. In the case of most kinds of injurious organisms their multiplication within the body unchecked would lead finally to its destruction. It is necessary, therefore, that the body have some means either of checking the development of the organisms or of neutralizing the poisons which they produce. The body has this power, and the machinery for it consists very largely of substances or structures in the body fluids. The detailed story of these is too complex to be told here. We shall content ourselves by saying that when an infection becomes established, as of scarlet fever, for example, the poisons that are produced are poured out by the organisms into the body fluids of the part where they happen to be located, and are taken up from there by the blood and distributed all over the body. The fever, headache, and other disagreeable symptoms are due to these poisons. The interesting thing about it is that the poisons themselves act toward the body as chemical regulators or hormones, exciting some or perhaps all of the cells of the body to a special kind of functional metabolism, which results in the manufacture of materials which neutralize the poisons. Thus, one of the very important properties of living protoplasm is to respond to the poisons from the metabolism of other cells by producing neutralizing material. Whether one dies from an infection or recovers from it depends on whether the cells are able to produce enough neutralizing material to prevent themselves from being killed or whether the poison is so abundant or so malignant that the cells are destroyed in spite of their activity in pouring out the neutralizing material.

Still another interesting thing about this whole matter is that every kind of infecting organism has its own kind of poison, which differs from that of the other kinds, and so the chemical effect of the poison upon the cells is not the same for one infection as for another. The functional metabolism of the cells in turn is adjusted to the kind of poison, so that the material they pour out is suitable to neutralize the particular poison which aroused them to activity in the first place, and in most cases no other. If one gets well from any infection, there is a surplus of the neutralizing material left in his body fluids, and, as long as it remains, he is secure from another infection of the same kind. This condition is defined as immunity. Since the neutralizing materials are different for different infections, immunity against one is in most cases of no avail against another. One may be immune against scarlet fever, but be just as likely to catch pneumonia as a person who has never suffered from any infection at all. It follows that an individual who has had and recovered from a great many infections has a correspondingly large assortment of neutralizing materials in his body fluids. Some of these appear to persist throughout life, others disappear fairly soon.

The next group of permanent constituents to be described consists of some materials which seem to have nothing at all to do as long as everything is going well: the body fluids bathing the cells, or, in the case of the blood, circulating about from part to part through the blood vessels. These come into play only when, as the result of injury, the fluids begin to escape; namely, in the case of bleeding, or, as it is technically called, hemorrhage. It is clear that if any injury is started sufficient to allow the fluid to escape, there would be no more reason why it should stop running out, unless prevented, than there is why water should stop running out of an open faucet of itself. Since, as we know, we do not bleed to death every time we get a slight cut, but after a long or shorter time the bleeding stops of itself, there must be some automatic arrangement by which the opening is plugged. All of us are familiar with the way in which this is done. We know that in the course of a few minutes after the bleeding begins, the blood tends to set into a firm jelly, which is called the clot. This clotting is the result of a chemical transformation which goes on in the blood as the result of its escape from the blood vessels and its exposure to the outside. The details of the chemical processes are too complicated to be described here; all we need to remember is that the blood within the body contains certain special materials which are soluble and therefore float in solution along with the other soluble materials. When any blood vessel is injured, so that the blood begins to escape, a series of chemical changes is started automatically by which this soluble material is changed and becomes insoluble, so that instead of remaining dissolved in the blood it is precipitated out. In this form it is called fibrin. Fibrin is a very sticky, stringy mass which forms a spongy network, spreads itself over the injury, and clings firmly to the edges, thus plugging the opening, unless the rush of blood is so strong that it keeps washing the fibrin away as fast as it is formed. Bleeding from small wounds will presently stop of itself, but if the hemorrhage is too great for this, it is necessary that the blood flow be slowed down or stopped artificially. This is done by locating and pressing upon the large blood vessel through which the blood is escaping. In order to do this successfully, one has to know something about the course of the blood vessels, and this will be described in a later chapter.

There are a few people whose blood lacks some of the necessary chemical substances to enable it to clot; such persons are known technically as “bleeders.” Even a slight injury in one of them will cause serious, or even fatal, hemorrhage, unless the escape of blood is stopped artificially, since it will not stop of itself. An interesting fact about this condition is that it runs in families; in other words it is hereditary.

In addition to all these constituents of the body fluids which are dissolved in them, there are in that part of the fluids confined to the blood vessels, which we call the blood, three kinds of structures floating; these we have next to describe briefly. The first of these are the red corpuscles. They give the blood its red color, although if looked at singly they appear yellowish rather than red. Red corpuscles are almost inconceivably tiny. They are red flexible disks, a little bit thinner in the middle than at the edges, about one three-thousandth of an inch in diameter. Some idea of the enormous numbers in the body can be gathered, when we say that a drop of blood the size of the head of a pin would contain four or five million of them. The red corpuscles are made up of a sort of framework of protein within which is inclosed a red coloring matter or pigment, known as hemoglobin. It is this pigment that gives the blood its color, and in some respects it is one of the most important of the nonliving substances in

the body. This is because it is the means by which the cells obtain sufficient supplies of oxygen. As we have already seen, every cell is constantly drawing from the body fluids about it the oxygen which is required for carrying on its metabolism. The fluids in turn get oxygen from the blood. It is necessary, therefore, for the blood to convey abundant supplies. Oxygen will dissolve in water, as is proven by the fact that fish and other aquatic animals are able to get enough oxygen from the water in which they live to serve their needs; but it is not sufficiently soluble to supply the needs of an active body like that of man. It is necessary, therefore, to have a special additional means of conveying oxygen besides its simply dissolving in the blood. This additional means is furnished by the hemoglobin, which is an “oxygen-carrying” pigment. What this means is that the hemoglobin has the property of taking up oxygen chemically, whenever it is exposed to a region where there is oxygen in abundance, and of giving it up again whenever it passes through a region where oxygen is scarce. It is thus that oxygen is conveyed from the lungs to the active tissues of the body. We shall have more to say about this in the chapter devoted especially to the matter of the oxygen supply.

We said a moment ago that the red corpuscles consist of a protein framework inclosing hemoglobin. They are not living. They must, therefore, have been made by living cells and poured out into the blood stream. We might suppose that this was done once for all and that the same red corpuscles are floating in our blood now that started floating there when the blood was first formed; but, as a matter of fact, this is not the case. There is a continuous breaking down of red corpuscles which must be made good by a continuous manufacture of new ones. Most of the larger bones in our bodies have a sort of spongy framework by which the ends, where the joints are, are made stronger. Within the space of these frameworks is a kind of marrow, known as red marrow, because it has such a very abundant blood supply. It is in this red marrow that the manufacture of red corpuscles goes on. There are throughout the red marrow living cells which are constantly dividing and subdividing, forming more and more so-called daughter cells. Within these daughter cells hemoglobin is presently deposited; a little later they lose the nucleus and probably the remainder of the living protoplasm as well, leaving just the framework of nonliving protein with its contained store of hemoglobin. This is the finished red corpuscle, and it breaks loose from the red marrow and floats out into the blood stream. The rate of manufacture of red corpuscles is very rapid; undoubtedly millions of them are formed daily in the various red marrow regions of the body. The total number of red corpuscles does not increase correspondingly, because they are broken down at the same rate as they are formed. It is now believed that in the spleen, which is a large organ of the abdomen whose function has always been obscure, those red corpuscles which are destined for destruction are picked out of the blood and broken down. We commonly suppose that this fate overtakes corpuscles that are worn out and are no longer efficient oxygen carriers, but we do not know, as a matter of fact, that the corpuscles do lose their effectiveness in course of time, nor have we any idea how the spleen could select out of the millions in the blood those particular ones which are no longer useful.

What the spleen does to the corpuscle is to break it up so that the protein and the hemoglobin in it are set free in the blood stream. We do not know what becomes of the protein; probably it is taken up and utilized. We do know that the hemoglobin is decomposed in the liver. One constituent of hemoglobin, in fact the constituent which gives it its ability to carry oxygen, is the element iron. Iron is not particularly abundant in living things, and we find that the body is thrifty with regard to it. When the liver decomposes the hemoglobin, the iron is saved in some way which enables the blood to carry it back to the red marrow, where it can be used over again. There are also some portions of the hemoglobin which are valuable as food material; the remainder, which is of no further use, is discharged from the body as a part of the bile.

Besides the red corpuscles the blood contains what are known as the white or, better, the colorless corpuscles. These, instead of being dead structures, as are the red corpuscles, are actual living cells, which float along in the blood stream or have the power of clinging to the walls of the blood vessels and crawling along them; in places where the blood vessels are very thin, they work their way right through, and so get out into the spaces among the tissues. There are not nearly so many colorless as red corpuscles, in fact, the latter are eight or nine hundred times as numerous as the former. A great deal of study has been devoted to colorless corpuscles to find out just what they do. We are by no means certain that we understand fully all their activities, but we do know that one very important thing that they do is to absorb into their own bodies tiny foreign particles that may be present in the blood stream or in the spaces among the tissues. There are not many kinds of foreign particles that can get into these places. In fact, about the only sort that can are the tiny plant or animal cells which are responsible for disease. When these foreign organisms invade the body, the colorless corpuscles may engulf them into their own bodies and destroy them. In this way a great deal of infection is prevented. Of course it may happen that the invading organisms are so numerous that the colorless corpuscles can not get rid of all of them, or the corpuscles themselves may show a diminished activity. It is an interesting fact that both the number of colorless corpuscles in the body and the vigor with which each colorless corpuscle attacks foreign organisms vary from time to time, so that we are much more secure against infection at some times than at others. In general we may say that when the body is in a condition of vigorous health the colorless corpuscles will be efficient. When we are run down, on the other hand, these cells share with all other body cells this state of low vitality. This explains why people who allow themselves to become run down are more likely to fall victims to infection than those who are in vigorous health, and emphasizes, of course, the importance of a habit of life that shall keep the body vigorously healthy at all times. It should be understood that the colorless corpuscles do not show the same effectiveness against all kinds of foreign organisms; some kinds of disease germs are able to bring about infection in the body quite regardless of the activity of the colorless corpuscles. From other kinds, on the other hand, they give the body very complete protection.

The most familiar example of the action of the colorless corpuscles is in the formation of what we all know as pus. There are a few kinds of organisms that, instead of getting into the body and becoming scattered through its fluids, establish themselves at certain points and by growth and multiplication accumulate at those places in large numbers. Examples are pimples and boils. In these cases the pus-forming organisms have located just under the skin and are multiplying there at a great rate. They produce poison which is absorbed from the place where they are and distributed through the body. This poison appears to have some sort of chemical attraction for the colorless corpuscles; at any rate the corpuscles gather from all around to the place where these organisms are located and engulf as many of them as they are able, but in so doing they themselves are destroyed, and pus, as we know it, is simply a mass made up of the dead bodies of the colorless corpuscles along with the organisms which they have destroyed and which in turn have destroyed them. In the ordinary pimple or boil the colorless corpuscles ultimately win the combat and the infection is completely overcome. It happens occasionally, however, that a pus-forming infection becomes established in some place where it manages to maintain itself in spite of the attacks of the colorless corpuscles. This happens very frequently in the tonsils, so that persons who have infected tonsils may have pus formation going on month in and month out. This may also take place at the roots of the teeth. In fact it is now commonly believed that whenever the nerve to the tooth dies, infection is certain sooner or later to become established at the tip of the root. These places where pus formation is going on continually are known as “pus pockets.” For a long time very little attention was paid to them. Persons occasionally suffered acute distress from gumboils or had attacks of sore throat owing to the infected condition of the tonsils, but beyond these no particular attention was paid to the pus-forming organisms, unless, as occasionally happened, an especially virulent type became established which brought on the condition commonly known as blood poisoning. Within the last few years the discovery has been made that there is a steady discharge of poison from every pus pocket into the body fluids and so to the blood stream. The amount is ordinarily so small that the body as a whole is not seriously affected, but now and then, either because of a larger production of poison or because of a lower resistance on the part of the body, serious ill effects are produced. Among them may be mentioned acute (inflammatory) rheumatism. This is not only an extremely painful condition, but it is very likely to leave serious after-effects, as, for example, injured heart valves, to give trouble for the rest of life. The discovery of these facts has given us great respect for pus pockets, so we no longer treat them carelessly. Infected tonsils are looked after, not simply because they bring about sore throat now and then, but even more because of the poison which they are likely to send through the body. It is probably not too much to say that the practice of dentistry has been revolutionized since the significance of pus pockets has been discovered. Formerly the killing of nerves to relieve aching teeth or to insure them against future aching was a common practice. The modern dentist, on the other hand, kills nerves to teeth with the very greatest reluctance and only as a last resort, because he knows full well that in so doing he is opening the way for the establishment of pus pockets with the train of ills that is likely to follow. At the present time methods of curing pus pockets at the base of the teeth are not very adequate, so that the extraction of the tooth has to be resorted to, but there is every reason to believe that the dental profession will shortly find methods by which pus pockets can be controlled without having to sacrifice the teeth.

The third kind of structures which are present in the blood stream are much smaller than the red corpuscles, but are nowhere near so numerous. They are called the platelets and are disk-shaped bodies composed chiefly of protein material, and probably, although not certainly, living cells. Their presence in the blood remained unsuspected up to about the end of the last century, not so much because of their very small size, as because they go to pieces very quickly after the blood is shed. By the time a drop of blood could be gotten under the microscope they would be all gone. They were discovered only as the result of the adoption of a method of treating the blood which preserved them long enough so that they would still be present when the blood was looked at under the microscope. They seem to have something to do with the changes that take place in the blood by which it is caused to clot. Whether this is their only function, we do not know.

We have now described the substances which are present in the blood and in the other tissue fluids as a fairly permanent part of their make-up. In addition to these there are present all the materials that are in transit to the cells or from them. These include all kinds of foodstuffs on their way from the digestive organs to the cells. As we shall see in detail later, the digestive organs work over the food that we eat before passing it on to the blood, so that the actual food materials that are being transported by the blood are the digested products of the food rather than the food itself. For example, there are to be found in the blood, in addition to the regular blood proteins which were described a moment ago, the digestion products of the food proteins on their way to serve the needs of the various cells of the body. There will be found also the digestion products of the other classes of foodstuffs. The body fluids contain also the waste products of cell metabolism on the way to be discharged from the body and the special products, such as the hormones, which are manufactured by certain cells and carried through the blood stream to act upon other cells. Of course we realize that not all of the materials that are manufactured by cells are poured out into the blood stream. Such materials as saliva, gastric juice and the like are passed directly from the cells in which they are manufactured into tubes by which they are conveyed to the digestive canal, where they carry on their special work of digesting the food.

Among the things which are present in the body fluids should be mentioned the two gases oxygen and carbon dioxide. We should expect oxygen to be present in the body fluids, because it is necessary for the metabolism of cells and can get to them only by being carried in the blood stream. We have seen in the red corpuscles the special method by which an abundance of oxygen is transported. Carbon dioxide is the gaseous product of the oxidation of carbon and is found in large amounts wherever there is burning, since carbon is the chief constituent of all fuel and whenever carbon is burned, carbon dioxide is formed. Since the fuel materials that are burned in the living cells consist largely of carbon, carbon dioxide is produced in them as well. They have to get rid of it, and the only way they can do so is by passing it out into the fluids that surround them, which in turn pass it on to the blood. The way in which the body handles these two gases makes up a special subdivision of the subject of physiology and will be so treated in the chapter on respiration.