=The capillaries= are small, thin, short, and very numerous. _They are very small_ so that they may go in between the cells of the tissues. _The capillaries are very thin_ so that the nourishment from the blood may pass readily into the tissues, and the waste material pass readily into the blood. _They are very short_ so that the friction may be less; and _they are very numerous_ so that all parts of the tissues may be supplied with blood, and that the blood may flow very slowly through them. Because of the number of the capillaries, their total volume is several hundred times larger than the volume of the arteries that empty into them, or of the veins that flow from them. Hence the blood flows slowly through the capillaries, as water flows slowly through a lake along the course of a river. All the changes between the blood and the lungs, and between the blood and the tissues, take place in the capillaries, and the object of the other parts of the circulation is merely to move the blood continually through the capillaries. =The effect of gravity= is to retard the flow in certain parts of the body and aid the flow in other parts, according to the position of the body (Exp. 2). =Fainting= is usually due to _lack of blood in the brain_, which in turn results from a weakening of the heart beat. Since the brain cannot work without fresh blood, fainting is accompanied by unconsciousness. Recovery from fainting is aided by loosening the clothing at the neck and by placing the head of the patient a little lower than the body so that the weight of the blood may aid the flow to the brain. Dashing a little cold water in the face shocks the nerves and arouses the heart to stronger beats. =The veins have valves= placed frequently along their course (Fig. 58). These valves are pockets made by a fold in the inner coat of the wall of the vein. When a boy places his hand in his pocket, the pocket swells out; but if he rubs his hand on the outside of the pocket from the bottom toward the top, it flattens down. So with the action of the blood upon the valves in the veins. (Repeat Exp. 6 in class.) [Illustration: FIG. 58.--VALVES IN VEINS. (Jegi.)]