118 THE HEART AND CIRCULATION. 



body, it does take place when a blood-vessel is injured or 

 when the blood comes in contact with the air, a wise pro- 

 vision 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 en- 

 dothelium, 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 ves- 

 sels 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 sub- 

 stance in the blood causes clotting. 



FIG. 45. Cells of blood: a, Colored blood-corpuscles seen on the flat; 6, 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 erythrocyles 

 which number about 5,000,000 to the cubic milli- 

 meter of blood, that the color of the blood is due. 

 Under the microscope they appear as small, spher- 

 ical, biconcave discs with a slightly greenish-yellow 



