=Blood-corpuscles.=--The solid parts of the blood are the red corpuscles, the white corpuscles, and the blood plaques or plates. It is to the _red corpuscles_, or _erythrocytes_ which number about 5,000,000 to the cubic millimeter of blood, that the color of the blood is due. Under the microscope they appear as small, spherical, biconcave discs with a slightly greenish-yellow color, which have a tendency to form in rouleaux. They are homogeneous, with no limiting membrane, and are made up of a fine network of tissue, the _stroma_, in which is embedded the hemoglobin or coloring matter. This hemoglobin is a crystalline body and the most complex substance known to chemists. The corpuscles are very flexible and can squeeze through small apertures, as in the tiny capillaries, and regain their shape. They are probably formed chiefly in the red bone marrow at the ends of the bones, which under the microscope shows red corpuscles in various stages of growth, and also in the spleen, for which no other use is known. Their function is to carry oxygen, which forms a chemical combination, though an extremely loose one, with the hemoglobin. As the tissues are more greedy of oxygen than is the hemoglobin, they rob the corpuscles of it. [Illustration: FIG. 46.--Various forms of leucocytes: _a_, Small lymphocyte; _b_, large lymphocyte; _c_, polymorphonuclear neutrophile; _d_, eosinophile. (Leroy.)] The _white corpuscles_ or _leucocytes_ are much fewer in number, about one to from 300 to 700 of the red, the average number being 5,000 to 10,000 to the cubic millimeter. They are larger than the red corpuscles, colorless, and spherical when at rest. Their structure is more definite, there being a definite cell substance or protoplasm and one or more nuclei, which vary more or less in shape and size. The corpuscles are classed in accordance with these variations in the nuclei. They are most numerous during digestion and are probably formed in the lymphatic system, constantly passing from the lymphatics to the arteries and veins. For they have the function of amœboid movement by which they not only wander from place to place in the blood, keeping close to the sides of the vessels, but pass through the walls of the capillaries, probably between the cells which form their lining, into the lymph spaces. This is known as _migration of the white corpuscles_. In inflammation they collect in the inflamed area to assist in allaying the inflammation by absorbing and carrying off its products. For they carry waste products and destroy poisons, acting as scavengers and protectors of the body. When they are unsuccessful and the inflammation gets the better of them, they become _pus corpuscles_. Besides the corpuscles there are seen floating in the blood small disk-like substances with no special characteristics, the _blood plaques_ or _plates_, whose function is unknown.