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



whale, where one would have naturally expected it in perfection, I could not 

 detect even a trace of it. 



What has been said of the semilunar valves of the pulmonary artery, may with 

 equal propriety be said of those of the aorta ; the only difference being that the 

 segments are stronger and more opaque, to harmonise with the greater strength 

 of the left ventricle. 



The Arterial or Semilunar Valves in Action. 



As the manner in which the semilunar valves are closed, does not seem to be 

 well understood, the following experiments conducted with various fluids and 

 liquid plaster of Paris, may prove interesting : — 



When the aorta is cut across two inches or so above the aortic semilunar 

 valve, and water introduced, the segments, if watched from beneath, are seen to 

 act with great alacrity, the smallest segment (Plate XXVIII. figs. 26, 27, and 28, 

 w), which is situated highest, descending with a spiral swoop, and first falling into 

 position ; the middle-sized segment (:r), which is placed a little lower, descending 

 in like manner, and fixing the first segment hy one of its lunulw or crescentic surfaces 

 (Plate XXVIII. fig. 26 a) ; the third and largest segment (v), which occupies a 

 lower position than either of the others, descending spirally upon the crescentic 

 margins {b c) of the other two, and wedging and screwing them more and more 

 tightly into each other. The spiral movement, as has been already explained, is 

 occasioned by the direction of the sinuses of Valsalva, which curve towards each 

 other, and direct the blood in spiral waves upon the mesial line of each segment 

 {wx'\i). 



It is well seen when liquid plaster of Paris is used, as the plaster, on setting, 

 enables the experimenter to examine the relations of the segments to each other 

 at leisure. Figures 27 and 28, Plate XXVIII., have been taken from specimens so 

 prepared. On removing one of the segments in such specimens, it is found to be 

 folded upon itself (Plate XXVIII., fig. 27 w), and to present two semilunar surfaces, 

 each of which is accurately applied to a corresponding and similar surface of that 

 segment of the valve which is next to it (Plate XXVIII. fig. 25 h V). The union, 

 therefore, between any two of the segments of a semilunar valve, is analogous in 

 many respects, to that occurring in a venous valve consisting of two segments. 

 There is, however, this difference ; in a venous valve, the segments, simply flatten 

 themselves against each other in the mesial plane of the vessel, to form a perpendicular 

 crescentic wall (Plate XXVIII. figs. 11 and 13 e) ; whereas in thes emilunar valves, 

 the segments in addition curve into each other^ and so form three perpendicular 

 crescentic walls, each of which radiates from the axis of the vessel (Plate XXVIII. 

 fig. 26 rso). In the venous valve, moreover, those portions of the segments 

 which come into apposition /orwz systemetrical crescents (Plate XXVIII. fig. 13 ^) ; 



