Valves differ from each other very con siderably in form, varying from a mere linear elevation on the inner surface, in which the probe scarcely hitches as it is pushed along, to others which almost form a long isosceles triangle. The outline of the margins deter mines the form of a valve. Cruveilhier says that the free margin is always straight ; but this is not the fact, though it is always more so than the attached.
In the first attempt at a valvular formation there is merely a slight elevation of the inner surface, and the free and attached margins are nearly parallel : these scarcely constitute valves. (Fig. 863. e, e'.) Valves, still imperfect but of larger dimen sions, are found at the orifices of veins, as well as in their canals. When existing in the former situation (and this is applicable to all valves thus placed), their attached margin corres ponds with the side of the orifice which is furthest from the heart, and the outline of this margin is determined by the form of the vessel's mouth. If the lesser vessel joins the greater at right angles, the opening will be circular, and the attached margin of the valve a semicircle. If, however, the junction be ob lique, and an acute angle be formed by the two vessels, then the margin attached will be drawn out of the semicircle, so as to form a sort of apex in the centre. The former of these conditions is shown at fig. 863. c, taken from a small valve at the orifice of an inter costal vein, joining the azygos; and the latter is represented at d, which is a drawing made from a valve at the orifice of an hepatic vein, where it joins the vena cava c' is an ideal section of the same. The dotted line in a,c and d represents, in conjunction with the line of the attached margin of the valve, the orifice of the tributary vein.
The next form, in order, and the one which is the most common, is the semi-lunar# valve, as it is ordinarily found in the tube of valved veins,—the attached margin being nearly semi circular, and the free nearly straight (fig. 863. b). It was probably of these valves that Haller spoke when he said that the attached margin of valves constitutes a parabolic curve: it may be applicable to some of the semi-lunar, but it cannot be to valves in general : a para bola is a constant form, but the outline of the valves is various. It appears to me that the
outline of a valve rather conforms to the ellipse, the curve being sometimes very much elongated, sometimes nearly approaching a circle. An ellipse is obtained from a cylinder, by an oblique section of it ; an oblique sec tion of a cylinder will only give an ellipse. For the line of attachment, therefore, of a valve with the cylindrical surface of a vein to generate an elliptical curve, that line of attach ment must coincide with an oblique section of the cylinder : and we see that this is fre quently the case, that is, when the valve is viewed in profile, its attached margin forms a right line at an angle, more or less acute, with the wall of the vein, the acuteness of the angle determining the elongation of the el lipse. This is represented infig.S64.B, where a shows the line of attachment seen in profile, and a' the curve generated by that line when seen in face. But sometimes the attached margin does not form, when viewed laterally, a straight line, but is more or less curved, as seen at fig. a, obi, and then we necessarily get an aberration from the ellipse, and an approxi mation, more or less near, to the parabola. Now, in such valves as are placed in pairs in the canals of veins, with their convex faces opposing and their cornua in contact, their relative length and breadth depend upon the obliquity of their attachment,— the require ments being constant, namely, that the op posed valves should meet by their free mar gins in a line across the centre of the vessel, for if the attachment be oblique and extend more in the axis of the vessel, it will require a much deeper valve to reach its fellow in the centre, than if the attachment were more across the tube, where a short, broad valve would accomplish the object. These two con ditions are represented in the figure (fig. S64.
A.), ‘s hick is an ideal section of a vein, in which both forms of valves are seen in a state of action.
Valves in the canals of veins, though in contact by their cornea, are sufficiently loose and large to fall back fully upon the sides of the vessels to allow the progressing current to pass on uninterruptedly.