194 Proceedings of the Royal Society 
to project boldly, and almost transversely, into the vein, it follows 
that the free margins of the segments run for a certain distance in 
a nearly parallel direction, the parallelism being maintained by the 
presence of a fibrous structure,* which projects from the wall of 
the vessel, and not only supports the free margins, but carries them 
fairly away from the sides into the interior. This relation of the 
dilatations to the segments, and of the segments to each other, has 
not hitherto been recognised, and shows that the union between the 
latter is not one of the margins simply, as has been supposed, but 
of the margins and a considerable portion of the sides ; the sides of 
the segments, on account of the angle at which they are set with 
reference to each other, being more and.more involved in proportion 
to the amount of pressure which is brought to bear upon them. 
The object of the dilatations is evidently threefold — Is^ (as has 
been stated), To allow the free margin of the segments to project 
into the interior, and maintain a position with reference to each 
other which necessitates their coming rapidly and naturally to- 
gether; 2dly, To increase the space occupied by the blood, and 
thereby extend the area, and the amount of pressure to which the 
segments are subjected ; and, '^dly (which is the most important). 
To determine the direction in which the blood shall act, that being 
from above downwards, and from without inwards. The effect of 
this arrangement on the segments is very obvious; these, when 
two exist, being forced together in the mesial plane of the vessel, 
where they mutually support each other ; and when three are 
present, in the axis of the vessel. In the latter case, the segments 
(from the more or less conical shape assumed by their free mar- 
gins while in action, and from the fact of their apices being free 
to move) rotate to a greater or less extent, while in the act of 
fixing or closing — the closure being effected by a combined wedge 
and spiral movement.f In the veins the closing of the valves may 
be regarded as purely mechanical, this being due not to any active 
power residing in the segments, hut to the pressure exerted upon 
them by the blood, and to the vital contractility residing in the 
The structure adverted to is very well seen in the arterial semilunar valves, 
t The spiral wedging movement is especially distinct in the segments of 
the arterial semilunar valves, 
