A valve the furthest removed in form from those first mentioned, is represented in fig. 863. a, which is the drawing of a valve situated at the orifice of the entulgent vein, where it joins the inferior cava in the sheep. Its depth is very considerable in proportion to its width, and the centre of its attached margin is nearly an acute angle, produced by the angle at which the renal vein joins the cava.* Structure of valves.—A venous valve con sists of a thin fibrous latnina, protruded into the tube of the vessel.
If the attached margin be carefully cut from the wall of a vein, and the organ be extended upon a slip of glass, it is seen to be thin at the free margin and thick at the attached ; if it be viewed by transmitted light, with the naked eye, it will be seen that the body of the valve is divided, rather indistinctly, into two portions, of which the free half is thinner and bluish-white, while the other is much thicker, and of a yellowish colour ; the line of demarcation, between these, extending across the body of the valve about parallel with the free margin ; this however, does not exist, in some valves.
The examination of a valve with the mi croscope requires that it should be completely unfolded at its free margin, in the neighbour hood of which, in the larger valves, its struc ture can alone be successfully observed. on account of the thickness and opacity of the other parts of the valve. When removed from a vein it should be placed upon a slip of glass, moistened, and, the attached margin being seized with forceps, it should be drawn over the surface of the glass so as to unravel any folds of the free margin, which are apt to occur.
The epithelium cannot always be found on the face of the valve,.and to see it the body must be very thin, and the focus of the in strument be thrown superficial to the fibrous lamina, when their nuclei will sometimes be viewed pretty distinctly. They are scattered evenly over the surface, but the cell-wall is extremely ill-defined, or not to be made out. At the margin of the valve they are frequently to be seen in one or two conditions,—free, detached nuclei, accidentally adherent along the margin ; or cells, seen edgewise, with con spicuous nuclei, as already described.
The fibrous lamina is the most conspicuous element of the valve, and constitutes its bulk. It consists almost entirely of white fibrous tissue, extending from side to side of the valve, running parallel with the free margin, and continuous, apparently, with the circular coat of the vein on each side of the attached margin of the valve. The fibres are singularly regular, and present the most beautiful and equal undulations, as if each fibre were of exactly the same size and extent ; and the un dulations of all succeeding fibres are precisely similar, as far as the tenuity of the structure towards its attached margin is sufficient 'to allow of its examination. These fibres may be looked upon as a portion of the circular coat protruded into the cavity of the vein.
Running at right angles to these, and placed upon a different plane, are to be seen fibres or elongated corpuscles of a spindle shape, and of very variable distinctness. These cor puscles are apparently muscular fibre-cells : they are very conspicuous in some specimens, and very indistinct in others ; whilst in others they cannot be discovered at all.
On treating a valve with acetic acid, two sets of nuclear corpuscles are to be discovered, running at right angles to each other. Those
which are parallel with the wavy fibres of the fibrous lamina are mainly oval, interspersed with a few more elongated ; and those, cutting the former at right angles, and being parallel with the muscular fibre-cells, are club-shaped and spindle-shaped, having a strong resem blance to the nuclei of muscular fibre-cells. The former, as it appears to me, are mainly persistent nuclei of areolar tissue, and belong to the wavy fibres, whilst the latter are evi dently muscular nuclei. Small threads of yel low fibre are occasionally seen. They are very fine, detached and scattered ; and fre quently exhibit a spiral form. In examining these structures it is necessary that, when removed from the body, they should not be allowed to dry before the observations are completed ; for the characters both of the fibrous lamina and nuclei are impaired per manently if they have once been dry.
is yet another item in the valvular apparatus of venous canals, — the oc currence of sinuses in the walls of the veins. These consist of small pouches or dilatations in the walls of the veins immediately in front of the valves. By these, the cylindrical form of the vessel is lost at that spot, two bulgings being apparent on it. These bulgings pro duce a certain amount of attenuation at these points, the thinness being in direct proportion to the increased area produced. The sinuses vary in dimensions, and do not bear any exact proportion to the size of the valve. Lateral dilatations may be seen in the walls of veins where no valves are present. (Seefig.1863., b', g, e', h.) When the valves are in action, these distended sinuses present knots along the course of the vessel ; an appear ance first described and figured by he observes of them, " non dissimilem for mam exterius pr se ferentes, ac nodi in plan taruni ranadis, et cultic apparent."• The supply of valves in the venous system is only partial, and is irregular. Valves are only found (in the human subject) in those veins which are subjected to muscular pressure, and are, therefore, most abundant in the veins of the limbs : and it has been well laid down by Cruveilhier, " that their pre sence and their number, their proximity and their distance from each other are directly influenced by the degree of opposition to the onward progress of the blood in any set of veins." t Of the veins of the head and neck, the ex ternal jugular is the only one with valves : it possesses two, which however are not suffi ciently compact to oppose injection. There are none in the cerebral sinuses or veins, and none in the internal jugular. The veins of the upper extremity are abundantly supplied with valves, which appear to be somewhat more numerous near the upper part of the arm. There are valves in the axillary vein, but none in the subclavian, vena innominata, or superior cava. They are abundant in the lower extremity, but most numerous at its lower part; in this respect differing from the superior extremity. The branches of the internal iliac vein are supplied with valves, while the external and internal iliacs them selves, the common iliac, and the inferior cava, have none. The spermatic veins (male) have valves, while the ovarian (female) are desti tute of them. In the azygos veins, Cruveil hier denies the existence of valves, but they are occasionally found in an imperfect state of development.