*Composition of the Blood.*—To the naked eye the blood appears as a thick but simple liquid; but when examined with a compound microscope, it is seen to be complex in nature, consisting of at least two distinct portions. One of these is a clear, transparent liquid; while the other is made up of many small, round bodies that float in the liquid. The liquid portion of the blood is called the _plasma_; the small bodies are known as _corpuscles_. Two varieties of corpuscles are described—the _red_ corpuscles and the _white_ corpuscles (Fig. 8). Other round particles, smaller than the corpuscles, may also be seen under favorable conditions. These latter are known as _blood platelets_. *Red Corpuscles.*—The red corpuscles are classed as cells, although, as found in the blood of man and the other mammals (Fig. 9), they have no nuclei.(6) Each one consists of a little mass of protoplasm, called the _stroma_, which contains a substance having a red color, known as _hemoglobin_. The shape of the red corpuscle is that of a circular disk with concave sides. It has a width of about 1/3200 of an inch (7.9 microns(7)) and a thickness of about 1/13000 of an inch (1.9 microns). The red corpuscles are exceedingly numerous, there being as many as five millions in a small drop (one cubic millimeter) of healthy blood. But the number varies somewhat and is greatly diminished during certain forms of disease. [Fig. 9] Fig. 9—Red corpuscles from various animals. Those from mammals are without nuclei, while those from birds and cold-blooded animals have nuclei. It is the _function_ of the red corpuscles to serve as _oxygen carriers_ for the cells. They take up oxygen at the lungs and release it at the cells in the different tissues.(8) The performance of this function depends upon the hemoglobin. *Hemoglobin.*—This substance has the remarkable property of forming, under certain conditions, a weak chemical union with oxygen and, when the conditions are reversed, of separating from it. It forms about nine tenths of the solid matter of the red corpuscles and to it is due the colors of the blood. When united with the oxygen it forms a compound, called _oxyhemoglobin_, which has a bright red color; the hemoglobin alone has a dark red color. These colors are the same as those of the blood as it takes on and gives off oxygen. The stroma, which forms only about one tenth of the solid matter of the corpuscles, serves as a contrivance for holding the hemoglobin. The conditions which cause the hemoglobin to unite with oxygen in the lungs and to separate from it in the tissues, will be considered later (Chapter VIII).