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