764 DR PETTIGREW ON THE RELATIONS, STRUCTURE, AND FUNCTION, 



introducing coloured plaster of Paris* into the vessel. I particularly direct 

 attention to this circumstance, as the thickenings referred to form fibrous zones 

 (Plate XXVIII. fig 6 /i), mhicli extend for a short distance into the substance of the 

 segments, and afford them a considerable degree of support. They further assist in 

 preserving the shape of the segments, and in enabling them to maintain the proper 

 angle of inclination — the said angle inclining the segments towards each other in 

 the mesial plane of the vessel (Plate XXVIII. figs. 3 and 4 e). When a valve, 

 consisting of two segments, is situated at the junction of a smaller with a larger 

 vein, one of the segments is usually placed between the two vessels at the point 

 of juncture (Plate XXVIII. fig. 11 J), the other on the wall of the smaller vein {a). 

 The position of the segments in such instances varies, their long diameter some- 

 times running parallel with the larger vessel, sometimes obliquely, but more 

 commonly transversely. When the valve consists of three segments (Plate XXVIII. 

 figs. 1 and 8, a be, r s t), the segments, as a rule, are unequal in size, one of them 

 being generally a little larger {t) than either of the other two (r s). They are semi- 

 lunar in shape, as in the smaller and middle-sized veins, and differ from the latter 

 in being less capacious. The tri-semilunar valves in the veins, may therefore be 

 regarded as intermediate between the fully developed bi-semilunar valves found 

 in the veins of the extremities, and the fully developed tri-semilunar valves which 

 occur at the origin of the pulmonary artery and aorta. The existence of valves 

 in the veins is indicated externally by a dilatation or enlargement of the vessel ; 

 the dilatation consisting of one, two (Plate XXVIII. figs. 3, 5, and 12 hg), or three 

 (Plate XXVIII. fig. 8 g a b) swellings, according as the valve is composed of one 

 two, or three segments. These dilatations or swellings are analogous to the 

 sinuses of Valsalva in the arteries (Plate XXVIII. figs. 17 and 18 d), their 

 direction in the veins of the extremities being from below upwards and from 

 within outwards. They form, with the segment to which they belong, open 

 sinuses or pouches which look towards the heart, and as they extend nearly as 

 far in an outward direction as the segments project inwardly, they give a very 

 good idea of the size and shape of the segments themselves. The only point 

 regarding the dilatations deserving of special attention, is the gradual thinning in 

 a direction from above downwards^ of those jyortions of the coats of the vessel 

 ivhich enter into their formation. The thinning referred to, is well seen when 

 vertical sections of the vessel are made, or when the vein is distended with 

 coloured plaster of Paris, as recommended. The swellings present a deeper 

 colour the nearer we approach to the attached border of the segments, the 

 attached borders, on account of their greater thickness, appearing as dense fibrous 

 zones (Plate XXVIII. fig. 5 a b, fig. 6 h). The object of the swellings is evidently 



* I have derived much information from the employment of this material ; its use having 

 enabled me to determine with something like accuracy, the relation of the segments of the valves to 

 each other when in action, and other points connected with the physiology of the heart. 



