=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? [Illustration: FIG. 63.--NETWORK OF FIBRIN IN HUMAN BLOOD (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 to life than a cut vein. Because of the toughness of the arteries, and because they are _located close to the bones_, they are less likely to be cut than the veins, which are softer and nearer the surface. The veins begin in capillaries and _empty into the auricles_; the arteries begin at _the ventricles_ and empty into capillaries; and there is a semi-lunar valve at the origin of each artery. =Cuts and Bruises.=--1. _Wash_ a cut under _running_ water. 2. _Stop the bleeding._ The washing in cold water may do this. Elevating an injured arm or leg will aid the blood greatly in forming a clot at the opening. 3. _Bandage_ firmly with a strip of cloth and sew the end. Keep wet the part of the bandage where the cut is; this lowers the temperature of the wound. It may be necessary to hold a gaping wound closed with strips of surgeon’s plaster placed across the cut. A handkerchief folded first into a triangle and then into a narrow bandage is often useful. A cut artery may be known from a cut vein by the brighter _color of the blood_, and by the flow being _stronger at each heart beat_, while the flow from a vein is uniform. Pressure to stop the flow of blood from an artery should be applied _between the cut and the heart_; but when the blood comes from a vein, the pressure should be applied _to the side of the cut farthest from the heart._ Apply hot water immediately for several minutes to a =bruise=. Either a bruise or a cut may be washed with a weak solution of some antiseptic such as carbolic acid. After washing a bruise it may be bound with a cloth soaked in witch hazel or arnica. THE LYMPHATIC SYSTEM