7(56 DR PETTIGREW OX THE RELATIONS, STRUCTURE, AND FUNCTION, 



of the segment, but transversely to the course of the vessel, and may be denomi- 

 nated the ho7'izontal Jih^es. 



Running in a precisely opposite direction, and confining themselves principally 

 to the body of the segment, are a series of equally delicate fibres (c), having a like 

 composition, and which, for the sake of distinction, may be described as the 

 vertical series. These two sets of fibres are superficial, and to be seen properly, 

 a power magnifying from 200 to 250 diameters is required. 



Radiating from the centre of the segment (Plate XXVIII, fig. \o e) towards its 

 attached harder (ii'), and seen through the more delicate horizontal and vertical 

 ones, is a series of stronger and deeper fibres, composed of white fibrous and yellow 

 elastic tissue, the former predominating. Still stronger and deeper than either 

 of the fibres yet described, and proceeding from the attached border of the 

 segment (Plate XXVIII. fig. 16 st), is a series of oblique fibres, continuous in very 

 many instances with corresponding fibres in the middle coat of the vessel. These 

 fibres cross each other with great regularity, and form the principal portion of the 

 segments. They are most strongly marked at the margin of the convex border of 

 the segment, where they form a fibrous zone or ring, which, as has been explained, 

 supports the segment, and carries it away from the sides of the vessel into the 

 interior. I have also detected, in the vicinity of the attached border of the 

 segment, some non-striped muscular fibres. The segment of a venous valve is 

 therefore a highly symmetrical and complex structure, the fibrous tissues com- 

 posing it, being arranged in at least three well-marked directions ; viz., horizontally, 

 vertically, and obliquely. The great strength which such an arrangement is cal- 

 culated to impart to the segment is readily understood. 



In conclusion, the segment is thinnest at its free margin, and thickest towards 

 its attached border ; the body being a little thicker than the free margins, and 

 where the extremities or narrow portions of the segments join each other, but 

 not so thick as the attached border. 



The Venous Valves in Action. 



The manner in which the venous valves act, is well seen when the vein is sus- 

 pended perpendicularly overhead, and water, oil, glycerine, or liquid plaster of 

 Paris, poured into it by an assistant from above ; the vein beneath the valve being 

 cut away, the better to expose the segments to the view of the spectator. When 

 the valve consists of one segment only, the fluid is observed to force it obliquely 

 across the vessel, and to apply its free crescentic margin to the interior or convex 

 surface with such accuracy as to prevent even the slightest reflux. When two 

 segments occur in the course of a vein, they are forced by the fluid simultaneously 

 towards each other in the mesial plane of the vessel (Plate XXVIII. figs. 3 and 12 e), 

 the sinuses {gh) behind the segments becoming distended, and directing the 



