An academic physiology and hygiene .. . l and opposesfurther backward flow. When a vein is thus pressedupon, several little knot-like swellings make their appear-ance at different points in its length, and these are the * 1 Huxley. THE BLOOD AND ITS CIRCULATION. 97 dilations of the walls above a valve which resists the back-ward pressure. In the veins of the extremities the valvesare especially numerous, since the weight of the columns of venous blood would tend to interfere with the onwardcirculation but for this provision. Each valve serves tocheck the slightest tendency toward a backward fl
An academic physiology and hygiene .. . l and opposesfurther backward flow. When a vein is thus pressedupon, several little knot-like swellings make their appear-ance at different points in its length, and these are the * 1 Huxley. THE BLOOD AND ITS CIRCULATION. 97 dilations of the walls above a valve which resists the back-ward pressure. In the veins of the extremities the valvesare especially numerous, since the weight of the columns of venous blood would tend to interfere with the onwardcirculation but for this provision. Each valve serves tocheck the slightest tendency toward a backward flow, andthus the work is distributed. 16. The veins are morenumerous than the arteries,and can retain nearly twice asmuch blood as the latter. Asa rule, they lie nearer the sur-face of the body than thearteries do, and are recogniz-able by their blue color. 17. The current of blood inthe veins is more rapid than ng. 35. showing the values of 1that of the capillaries, but slowerthan the arterial current. Theflow through the veins is con-. EX PL AN ATI ON. a, part of a vein laid open, withtwo pairs of valves. B, longitudinal section of a vein, tinned by the pressure of the showing the valves dosedblood in the capillaries, the con- ..,(:I,,,rti,,n °< a ***** !ein;ex 1 hibiting a swelling at a pair of van traction of muscles through which the veins pass, the suction power of the auricles,and by the expansion of the chest in breathing, whichtends to draw the blood into the lungs. 18. Commencing in the capillaries, as we have learned,the veins unite into larger branches, and these into stilllarger ones, all converging toward the heart as do thebranches of a tree towards its trunk. Finally they allome united in two great veins, the vena cava descend-ing, formed by the veins of the head and arms, and the 98 ACADEMIC PHYSIOLOGY. vena cava ascending, formed by those of the legs andtrunk. These two great trunks connect with the rightauricle of the heart. 19. Vaso-Motor Nerves. — Vaso
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