. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 1378 VEIN. in the middle of the vein, and are there united by the contact of their margins ; and so accurate is the adaptation, that neither by the eye, nor by any other means, can the slightest chink along the line of contact be perceived. But if the probe be now intro- duced from the extreme towards the more central parts, the valves, like the flood-gates of a river, give way, and are most readily pushed ;* Valves exist in two different situations ; namely, at the orifices of lesser veins where they join the
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. 1378 VEIN. in the middle of the vein, and are there united by the contact of their margins ; and so accurate is the adaptation, that neither by the eye, nor by any other means, can the slightest chink along the line of contact be perceived. But if the probe be now intro- duced from the extreme towards the more central parts, the valves, like the flood-gates of a river, give way, and are most readily pushed ;* Valves exist in two different situations ; namely, at the orifices of lesser veins where they join the trunks which they supply, and in the canals of veins, arranged at various points. Fig. * Diagram exhibiting different forms of Valves. a, valve placed at the orifice of the renal vein in the sheep, seen in face; a', the same in ideal section ; b, the ordinary semilunar valve from the tube of a vein ; I/, the same in section ; c, imperfect valve at orifice of intercostal vein ; c7, section of imperfect valve at orifice of hepatic vein; d, the same in face (the dotted lines in a, c and d correspond to the orifices of the respective veins); e, very imperfect valve; ef, the same in section ; f, plan of double valve at orifice of vein ; /', the same in section ; g, ideal section of small valve and sinus; h, sec- tion of sinus without a valve. At the orifices of veins the valves are either single or double: when single, the free margin always looks towards the heart. In the canals of veins the valves are usually double in the larger, and single in the smaller. It is rare, in the human subject, to find them in threes * Loc. cit. on the same plane, though it has been found both by Morgagni and Haller ; but in the great vessels of the larger mammalia it is common. There are none in the capillaries; but, accord- ing to Henle, in veins of not more than half a line in diameter, they make their appear- ance. For the purposes of anatomical description a valve may be said to have a bo
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