At first the clot includes the whole mass of blood, takes the shape of the vessel in which it is contained, and is of a uniform color. But in a short time a pale yellowish fluid begins to ooze out, and to collect on the surface. The clot gradually shrinks, until at the end of a few hours it is much firmer, and floats in the yellowish fluid. The white corpuscles become entangled in the upper portion of clot, giving it a pale yellow look on the top, known as the _buffy coat_. As the clot is attached to the sides of the vessel, the shrinkage is more pronounced toward the center, and thus the surface of the clot is hollowed or _cupped_, as it is called. This remarkable process is known as coagulation, or the clotting of blood; and the liquid which separates from the clot is called serum. The serum is almost entirely free from corpuscles, these being entangled in the fibrin. Illustration: Fig. 67.—Diagram of Clot with Buffy Coat. A, serum; B, cupped upper surface of clot; C, white corpuscles in upper layer of clot; D, lower portion of clot with red corpuscles. This clotting of the blood is due to the formation in the blood, after it is withdrawn from the living body, of a substance called fibrin.[31] It is made up of a network of fine white threads, running in every direction through the plasma, and is a proteid substance. The coagulation of the blood may be retarded, and even prevented, by a temperature below 40° F., or a temperature above 120° F. The addition of common salt also prevents coagulation. The clotting of the blood may be hastened by free access to air, by contact with roughened surfaces, or by keeping it at perfect rest. This power of coagulation is of the most vital importance. But for this, a very small cut might cause bleeding sufficient to empty the blood-vessels, and death would speedily follow. In slight cuts, Nature plugs up the wound with clots of blood, and thus prevents excessive bleeding. The unfavorable effects of the want of clotting are illustrated in some persons in whom bleeding from even the slightest wounds continues till life is in danger. Such persons are called “bleeders,” and surgeons hesitate to perform on them any operation, however trivial, even the extraction of a tooth being often followed by an alarming loss of blood. Experiment 86. A few drops of fresh blood may be easily obtained to illustrate important points in the physiology of blood, by tying a string tight around the finger, and piercing it with a clean needle. The blood runs freely, is red and opaque. Put two or three drops of fresh blood on a sheet of white paper, and observe that it looks yellowish. Experiment 87. Put two or three drops of fresh blood on a white individual butter plate inverted in a saucer of water. Cover it with an inverted goblet. Take off the cover in five minutes, and the drop has set into a jelly-like mass. Take it off in half an hour, and a little clot will be seen in the watery serum.