OF THE VALVES OF THE VASCULAR SYSTEM IN VERTEBRATA. 765 



twofold,— :/?rs^/z/, to cause the blood to act on the segments of the i^alve from above 

 downwards, and from mitliout inwards, in the direction of the mesial plane^ or of 

 the axis of the vessel, according as there are two or three segments present; 

 and, secondly, to increase the area over which the pressure exerted by the reflux 

 of the blood extends. When the vein is opened between the segments, when two 

 are present, each of the segments is seen to form two curves, — one curve giving 

 its concave or free margin (Plate XXVIII. fig. 4 c), the other its convex or 

 attached one {a) ; but when the section is carried through the wall of the vein, and 

 through the centre of one of the segments, one curve only is obtained (Plate 

 XXVIII. fig. 3 g) ; and it is useful to remember this, as it shows with what facility 

 the structures entering into the formation of the segments, viz., the lining mem- 

 brane of the vessel and certain parts of the middle and internal coats, are given 

 off. It also shows how the lower portion of the dilatation (g), while it supports 

 a certain quantity of the refluent blood, guides by far the greater quantity on to 

 the valves (b), rendering their closure not a matter of accident, but of necessit3^ 



The segments of the venous valves are exceedingly flexible, and so delicate as 

 to be semi-transparent. They possess great strength and a considerable degree 

 of elasticity.* Usually they are described as consisting of a reduplication of the 

 fine membrane lining the vessel, strengthened by some included fibro-cellular 

 tissue, the whole being covered with epithelium. This description, however, is 

 much too general to convey any very accurate impression of their real structure, 

 and the following, drawn up from the examination of a large number of specimens 

 taken from man, the horse, ox, sheep, and other animals, may prove useful. 



When one of the segments of a well-formed bi-semilunar valve removed from 

 the human femoral vein, is stained with carmine, fixed between two glasses, and 

 examined with the microscope or pocket lens, the subjoined phenomena are wit- 

 nessed : — 



1st, The lining membrane of the vessel covered with epithelium is seen to 

 form the investing sheath of the segment, no breach of continuity being any- 

 where perceptible. 



2d, Large quantities of white fibrous tissue, mixed up with areolar and 

 yellow elastic tissue, from the middle and internal coats of the vessel, are observed 

 to extend into the segment. The fibres composing these tissues pursue a definite 

 arrangement. 



Thus running along the concave or free margin of the segment (Plate 

 XXVIII. fig. 14 a), as likewise on the body, especially where the segments join 

 each other (b), are a series of very delicate fibres, consisting principally of 

 yellow elastic tissue. These fibres proceed in the direction of the long diameter 



* Hunter denies the elasticity of the segments, on the ground that the valvular membrane is 

 not formed of a reduplication of the lining membrane of the vessel, an opinion at variance with recent 

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



VOL. XXIIL PART III. 9 T 



