VEIN. 



1379 



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 injfig-.864.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^g. B, b b', and then we necessarily get 



Fig. 864. 



I' 



Diagram exhibiting the Attachment and consequent 

 Forms of Valves. 



A, section of valves in action, , elongated ellip- 



tical valve ; b, broad semilunar valve. 



B, exhibits the lines of attachment of, a, elliptical 

 valve in profile, and a', the resulting form ; b, the 

 attachment of parabolic valve in profile, and V, 

 the resulting form. 



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 (Jig. 864. 



A.), which 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 cornua, are sufficiently loose 

 and large to fall back fully upon the sides of 

 the vessels to allow the progressing current 

 to pass on uninterruptedly. 



A valve the furthest removed in form from 

 those first mentioned, is represented in Jig. 

 863. a, which is the drawing of a valve situated 

 at the orifice of the emulgent 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 lamina, 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. 



Thefibrout 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 



* The extreme depth of the valve here represented 

 is not constant ; it sometimes barely reaches across 

 the orifice. 



4 T 2 



