Human physiology / by William Furneaux.

  • William Samuel Furneaux
Date:
1909
Catalogue details

Licence: In copyright

Credit: Human physiology / by William Furneaux. Source: Wellcome Collection.

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    of this network varies considerably in different parts. The walls of capillaries are exceedingly thin, so that fluids very readily ooze through them, and, on this account, the changes which the blood undergoes during its circulation take place chiefly in these vessels. After the blood has passed through the meshes of the capil- •laries, it is collected up by very small veins. These small veins unite, forming larger and larger veins, till at last they open into the great veins which take the blood direct into the auricles. The structure of veins is very similar to that of arteries. Their walls are composed of the same three coats, but they are thinner, and the muscular and elastic fibres are not nearly so abundant. Hence the walls collapse when the veins are empty. M h Fig. 122. - Capillary Blood- vessels in the Web of a Frog’s Foot, as seen with the microscope. a, small artery ; 6, capillaries ; c, small vein. The arrows show the course of the blood. B c Fig. 123.—Diagram showing the Valves of Veins. A, part of a vein laid open, with two pairs of valves. B, longitudinal section of a vein, showing the valves closed. C, portion of a distended vein, ex- hibiting a swelling at a pair of valves. Most veins are also provided with valves like the semilunar valves of the heart, sometimes arranged at very short intervals. All these valves are situated with their free edges turned toward the heart. Hence they allow the blood to flow freely towards that organ ; but any backward tendency on the part of the blood would fill these pouch-like valves, causing them to extend across and to close the vein. In small veins the valves are sometimes single, but generally they are arranged in pairs. Arteries are generally deeply set, while many of the larger veins are situated very near the surface of the body, so near that their direction, as shown by the bluish tint visible through the skin, may be easily traced. If we lay bare the arm, we notice several of these veins on the front surface, and by pressing the
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