OF THE VALVES OF THE VASCULAR SYSTEM IN VERTEBRATA. 777 



whereas in the semilunar one, the surfaces referred to, are non-symmetrical ; in 

 other words, the horns of the crescents forming the lunulse, are broader towards the 

 mesial line of the segments (Plate XXVIII. fig. 20 c^ than where they meet above 

 (b). As a result of this want of symmetry in the lunulee or opposing surfaces of 

 the semilunar valves, the apices or central portions of the segments come together 

 in the axis of the vessel throughout a considerable space (Plate XXYIII. fig. 25 x), 

 and form a union of the most perfect description. The extent of the union increases 

 in the inverse of the pressure applied (compare dotted lines, Plate XXVIII. fig. 

 25 m m' with plain ones b b'\ and is rendered very secure from the segments 

 being wedged into each other in a direction from above downwards and from with- 

 out inwards (Plate XXVIII. fig. 26 vwx). Retzius,* who figures the manner 

 of closure of the semilunar valves, does not seem to have been aware of this fact, 

 for he represents the segments as coming together in the axis of the vessel at 

 three points, an arrangement which could scarcely fail to occasion a certain 

 amount of regurgitation. If the closure of the semilunar valves be watched from 

 above, other phenomena are observed. When, for example, the aorta and semi- 

 lunar valve of the whale were sunk in water and permitted to remain un- 

 disturbed, the thicker portions of each segment were seen to project in an upward 

 and inward direction, the free margins being by this arrangement brought more 

 or less closely into contact, and supported on a level corresponding to the top of 

 the sinuses of Valsalva. When, however, the preparation was raised in the 

 vessel, so that the water acted from above on the central and more unsupported 

 portions of the segments ; the free margins, together with the more moveable parts 

 of the bodies, descended to the extent of fully an inch and a half In so doing, 

 the free margins of the segments were projected against and accurately applied 

 to each other, clearly showing that the fluid, because of its weight and the spiral 

 downward and inward direction communicated to it by the sinuses of Valsalva, 

 is sufficient to effect the closure. When the closure was taking place, the seg- 

 ments fell into position in rotation, but at so nearly the same interval of time, 

 that they mutually regulated the amount of downward and inward movement ; 

 and so prevented each other from protruding too far into the interior of the vessel. 

 When the hand was introduced into the aorta, which the great size of the speci- 

 men! readily permitted, and one of the segments was pushed in an outward 

 direction, it was found to apply itself to the sinus of Valsalva behind it, with 

 more or less accuracy ; the extremities of the segments, where they unite above, 

 projecting to form three ridges, which are sjnrally inclined with reference to each 

 other, and are no doubt useful in directing the blood into the aorta proper. From 

 the foregoing description of the venous and arterial semilunar valves in mam- 



* Om Mekanismen af Semilunar Valvlernes tillolutning. 



t In this case, the aorta had a girt of 27 inches ; the average size of the segments being 

 9 inches by 7. 



VOL. XXin. PART III. 9 Y 



