OF THE VALVES OF THE VASCULAE SYSTEM IN VERTEBRATA. 763 



figs. 3 and 4 a J) ; the convex border being attached to the wall of the vessel 

 obliquely (Plate XXVIIL fig. 5 a h\ the crescentic or concave margin, which is 

 free (Plate XXVIIL fig. 4 c), and directed towards the heart, projecting into the 

 vessel. When one segment constitutes the valve, and it occurs in the course of 

 a vein, it is placed obliquely in the vessel (Plate XXVIIL fig. 9), its attached 

 convex border {a) occupying rather more than a half of the interior. When the 

 segment occurs at the junction of a smaller with a larger vein, its convex border 

 is attached to a half or more of the orifice of the smaller one where it joins the 

 larger, its free margin running transversely to the larger trunk. In such cases 

 the segment acts as a moveable partition or septum, common alike to both vessels, 

 but its position and relations are such, that while it readily permits the blood 

 from the smaller vein to enter the larger one, it effectually prevents its 

 return. 



When the valve consists of two segments, they are semilunar in shape, and 

 very ample, the vertical measurement of each, being not unfrequently nearly 

 twice that of the diameter of the vessel itself (Plate XXVIIL figs. 3 and 4 a ^). In 

 such cases both segments are usually of the same size, so that they divide the 

 vessel into two equal parts {e)» They are placed obliquely with regard to each 

 other (Plate XXVIII. fig. 5 a b), their convex borders, which are attached to the 

 interior of the vessel, starting from a common point above (Plate XXVIII. fig. 2 i), 

 and gradually diverging {e) to curve round and reunite on the opposite side of the 

 vessel (d) ; their concave and free margins inclining towards each other (Plate 

 XXVIII. fig. 12 e), and being directed, as in the more simple valve, towards the 

 heart. The free margins of the two segments, like the attached ones, start from 

 a common point (Plate XXVIII. fig. 4 r), but such is the shape of the segments, 

 and such the angle at which they are placed with regard to each other, that they 

 do not diverge to the same extent, but run moi^e or less p>arallel* This relation 

 of the segments to each other above, is in part accounted for by the presence of 

 a fibrous structure (Plate XXVIII. fig. 4 r), mliicli extends from the mall of the 

 vessel into the interior, and supports them at a certain distance from the sides 

 of the vessel. The fibrous structure referred to is well seen in the semilunar 

 valves of the pulmonary artery and aorta (Plate XXVIIL fig. 36 n n'), and seems 

 to have escaped observation. In a line corresponding to the attached border of 

 each of the segments (Plate XXVIII. fig. 4 a b), the middle and inte7'nal coats 

 of the vein are thickened, as may be ascertained by a vertical section, or by 



reference to photographs 1, 2, 3, and 4, Plate XXVIII. The edges referred to are least curved when 

 the valve is distended or in action (Plate XXVIII. fig. 13 e), but the curve is never altogether 

 absent. — Treatise on the Blood, pp. 181, 182. 



* I was much struck, on injecting the external saphenous vein of the human subject from the 

 dorsum of the foot, to find, on dissection, that the free margins of some of the segments were in 

 contact throughout ; clearly showing, that when the segments are allowed to float in a fluid, they are 

 so projected against each other, that even the slightest reflux will instantly close them. 



