body, it does take place when a blood-vessel is injured or when the blood comes in contact with the air, a wise provision of nature, as otherwise the tendency would be for bleeding to go on indefinitely after injury. The greater the surface with which the blood comes in contact the more quickly it clots. Injury to the vessel wall itself is necessary; the endothelium must be cracked. Under extreme injury the muscular coat of the vessel undergoes spasmodic contraction and partially closes it. Hence a wound caused by tearing is less likely to bleed than one due to cutting.
The valves of the heart, which are covered with endothelium, are frequently the seat of fibrin coagulation, bits of the fibrin thus formed giving rise to conditions in various kinds of heart trouble. Or the bits of fibrin float in the blood and perhaps lodge in the small vessels of the brain and cause apoplexy. Pus in various parts of the body will set up coagulation in nearby arteries. In fact, the presence of any foreign substance in the blood causes clotting.
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Fig. 45.—Cells of blood: a, Colored blood-corpuscles seen on the flat; b, on edge; c, in rouleau; d, blood platelets. (Leroy.)
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
