The fibrous mass of which the vein is com posed, in many places exhibits the appearance as if the fibres were formed of spindle-shaped cells strung together, with their ends over lapping ; and these cells may be occasionally isolated,—they are spindle-shaped and have an oval nucleus. They resemble those ob tained from the middle coat of the aorta of a fcetal pig, by Lehmann. (See his figure.) Whether this condition is the result of im perfect maturation of the tissue of which the vein is composed, or otherwise, I am unable to say.
V. Valves. — The valves are membranous folds on the inner surface of the veins, having a definite form and regular arrangement with regard to their object—the progress of the venous blood to the heart, and obstruction to its regurgitation. They are of peculiar interest to the physiologist, as presenting a clear and elegant specimen of animal mechanics; and to the literary anatomist they are not less inter esting, as having been one of the main objects which suggested to Harvey his brilliant de duction of the circulation of the blood.
Cruveilhier states that the valves of veins were first discovered by Etienne. Harvey leaves the priority of discovery in doubt; for he writes, " The celebrated Hieronymus Fa bricius of Aquapendente, a most skilful ana tomist and venerable old man ; or, as the learned Riolan will have it, Jacobus Sylvius, first gave representations of valves in the veins."* Fabricius himself lays claim to the discovery, "non solum nulla prorsus mentio de ipsis facta sit, sed neque aliquis prius hmec viderit, quam anno Domini septuagesimo quarto, supra millesimum et quingentesimuin, quo a me summa cum lxtitia inter disse candum observata fuere."t Marx, who ap pears to be profoundly versed in the literature of this subject, says the discovery is due to Erasistratus ; for he observes, " Erasistratus (304 B.c.), item clarus anatomus jam subtilius in structuram et usum venarum inquisivit, valvulas jam observavit."* Harvey's description of the valves is so apposite and clear, as well as so interesting in a literary point of view, that I shall quote it. The valves, he observes, " consist of raised or loose portions of the inner membrane of these vessels, of extreme delicacy, and a sig moid or semilunar shape. They are situated at different distances from one another, and diversely in different individuals ; they are connate at the sides of the veins ; they are directed upwards or towards the trunks of the veins ; the two—for there are, for the most part, two together —regard each other, mu tually touch, and are so ready to come into contact by their edges, that if any thing at tempt to pass from the trunks into the branches of the veins, or from the greater veins into less, they completely prevent it ; they are further so arranged, that the horns of those that succeed are opposite the middle of the convexity of those that precede, and so on, alternately." He further writes, " In many
places two valves are so placed and fitted, that, when raised, they come exactly together 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 he 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 aside."# 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.
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 then] in threes 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 body, margins, and cornua. The body of the valve has a cardiac, concave, or proximal face and a convex or distal face. The margins are,—the free, or that which is unattached, and the attached margin. The cornua are the angles formed by the meeting of the two margins, and constitute the extreme lateral boundaries of the valve.