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separated from serum.

Some impurities and a large amount of water escape from the blood as it passes through the skin.

Coagulation.—So long as blood is in an uninjured blood vessel it remains a liquid. In a few minutes after it flows from a blood vessel, it forms into a stiff, jellylike mass called a clot (Fig. 62). The process of forming the clot is called coagulation, and it is brought about by the albuminous substance called fibrin, which is always in the plasma of healthy blood. On exposure to air the fibrin forms into a network of fine threads throughout the mass (Fig. 63) and the corpuscles become entangled in the meshes. The clot consists of the fibrin of the plasma and corpuscles; the watery portion of the plasma, called the serum, separates from the clot (Fig. 62). The property of coagulating is a great safeguard, as a clot often plugs up a cut blood vessel. What is the difference between serum and plasma?

(enlarged).

Veins and Arteries compared.—The veins have thin, soft walls and the arteries have thick, tough, elastic walls. When a vein is cut, it may usually be closed by pinching the walls of the end together. If an artery is cut, the walls will not readily stick together, but often stand open until the end of the artery is tied. For this reason, and because an artery is subject to the direct pressure of the heart, a cut artery is more dangerous