OF THE VALVES OF THE VASCULAR SYSTEM IN VERTEBRATA. 767 



current and regulating to a certain extent the amount of pressure. The closure in 

 this instance is almost instantaneous, and so perfect that not a single drop 

 escapes. It is effected by the free margins of the segments, and a, large proportion 

 of the sides ^ coming into accurate contact, the amount of contact increasing in the 

 inverse of the pressure applied. If liquid plaster of Paris be used for distending 

 the vein, and the specimen is examined after the plaster has set, one is struck 

 with the great precision with which the segments act (Plate XXVIII. figs. 6 and 

 W ah), these coming together so symmetrically, that they form by their union a 

 perpendicular mall or septum (Plate XXVIII. figs. 6, 11, and 12, e) of a beauti- 

 fully crescentic shape* (Plate XXVIII. fig. 13 e). This fact is significant, as it 

 clearly proves that the concave or free margins of the segments, and a consider- 

 able proportion of the sides, run parallel to each other when the valve is in action, 

 a circumstance difficult of comprehension, when it is remembered that the convex 

 borders of the segments are attached obliquely to the walls of the vessel, and that 

 the segments, when not in action, incline towards each other at a considerable 

 angle. The very accurate apposition of the segments, when the valve is closed, is 

 to be traced : — - 



] St, To the direction and shape of the venous sinuses, which conduct the fluid 

 employed, on to the segments in almost equal quantities. 



2d, To the disposition of the free margins of the segments, which, as was 

 explained, run side by side, and are supported by fibrous structures which 

 carry them away from the sides of the vessel for some distance ; and, 



Zd, To the amplitude of the segments themselves, which allows them to 

 come together without difficulty, and when the pressure is applied, to flatten them- 

 selves against each other to form the perpendicular crescentic wall adverted to. 



In the event of two segments occurring at the entrance of a smaller into a 

 larger vein, one of them being situated at the junction of the smaller vessel with the 

 main trunk (Plate XXVIII. fig. 11 b), the other on the Avail of the tributary branch 

 («) ; the former, i.e., the common or septal segment, is forced by the fluid in a 

 slightly outward direction, the latter in an opposite or inward direction, the free 

 margins and sides of the segments being by this arrangement accurately applied to 

 each other to form an impervious wall or septum {e) as already described. When 

 the entrance of a smaller into a larger vessel is guarded by two segments situated 

 on the tributary branch at its orifice, their action is precisely the same as when 

 they are placed in the course of a vein. When three segments are present, as 

 happens in the larger trunks, the closure is effected in a manner greatly re- 

 sembling that by which the semilunar valves of the pulmonary artery and aorta 



* In order to see the perpendicular wall formed by the flattening of the sides of the segments 

 against each other when the valve is in action, the vein and the plaster should be cut across im- 

 mediately above the valve, and the segments forcibly separated by introducing a thin knife between 

 them. In fig. 13, Plate XXVIII., one of the segments has been quite removed. 



