OF THE VALVES OF THE VASCULAR SYSTEM IN VERTEBRATA. 771 



the requisite degree of support, but in carrying them away from the sides of the 

 vessel, and in inclining them towards each other at such an angle as insures that 

 their free margins, especially where they unite above (Plate XXVIII. jBg. 86 h c), 

 shall be more or less parallel when the valve is in action. That the frame- 

 works afford the support here indicated, is proved by the fact, that when liquid 

 plaster of Paris is introduced into the ventricles, and forced through the pul- 

 monary artery and aorta, in the direction of the circulation, the attached borders 

 of the segments do not fall back into the sinuses of Valsalva (Plate XXIX. 

 figs. 50 and 51 v w) to the same extent as the sides and free margins, but project 

 so as to furnish the casts thus obtained, with corresponding depressions (rs). 

 The sinuses of Valsalva are formed above by the dilatations or expansions 

 of the great vessels, and the one occupies a higher position than either of the other 

 two. They are further unequal in size (Plate XXVIII. fig. 26 w cc v\ the 

 highest and smallest occurring anteriorly, that which is intermediate in size being 

 placed posteriorly, while the lowest and largest is directed towards the septum. 

 They correspond in situation and dimensions to the segments behind which they 

 are found, and differ from the venous sinuses in being more capacious, a section of 

 the sinus and its segment (which is likewise very ample) giving a sweep of nearly 

 half a circle (Plate XXVIII. fig. 18 bis). As a result of this amplitude, those 

 portions of the segments which project into the vessel are, during the action of 

 the valve, closely applied to each other throughout a considerable part of their 

 extent (Plate XXVIII. fig. 26 a be); the great size of the sinuses furnishing 

 an increased quantity of blood for pressing the segments from above downwards, 

 and from without inwards, or in the direction of the axis of the vessel. The 

 sinuses o/ Valsalva curve towards each other in a spiral direction; and this ought 

 to be attended to in speaking of the action of the semilunar valves, as the sinuses 

 direct the blood spirally on to the mesial line of each segment (Plate XXVIII. 

 fig. 26 V ov x), and cause the segments to twist and wedge into each other, as re- 

 presented at -y ra ^ of figs. 26, 27, and 28, Plate XXVIII. In order to determine 

 this point, I procured a fresh pulmonary artery and aorta, and after putting the 

 valves into position with water, caused an assistant to drop liquid plaster of Paris 

 into the vessels. The greater density of the plaster gradually displaced the 

 water, and I was in this way furnished with accurate casts of the sinuses and of 

 the valves. The segments of the semilunar valves, unlike the venous ones, are 

 almost invariably three in number.* They differ in size and in position, and 

 in this respect resemble the sinuses of Valsalva, to the inside of which they 

 are found. Thus the segment which is smallest is situated anteriorly, and 

 occupies a higher position than either of the others ; that which is second in 



* Dr John Hughes Bennett speaks of a case in which foui- were present, but whether the addi- 

 tional segment was congenital, or the result of disease, is not easy to determine. '• Principles and 

 Practice of Medicine," 1858, p. 550. 



