768 DR PETTIGREW ON THE EELATIONS, STRUCTURE, AND FUNCTION, 



are closed ; the fluid employed, in virtue of the direction given to it by the 

 venous sinuses, causing each of the segments (Plate XXVIII. fig. 8 rst) to hend 

 or double upon itself at an angle (i) of something like 60°;* the three lines formed 

 by the doubling and union of the three segments dividing the circle corresponding 

 to the wall of the vessel, into three nearly equal parts. In the doubling of the 

 segments upon themselves, each segment regulates the amount of lending which 

 takes place i7i that next to it, and as the free margins of the segments so bent 

 advance synchronously towards the axis of the vessel, they mutually act upon 

 and support each other. As the three segments are attached obliquely to the 

 wall of the vessel, while the free margins, after the folding has taken place, are 

 inclined towards and run parallel to each other (« V), they form an inverted 

 dome consisting of three nearly equal parts, the margins of the segments, and a 

 certain portion of the sides, when the pressure is applied, flattening themselves 

 against each other to form three crescentic partitions or septaf which run from 

 the axis of the vessel towards the circumference. 



The tri-semilunar valve, as will be seen from the foregoing explanation, is 

 closed in a very different manner from the bi-semilunar one. The occlusion of 

 the vessel, however, is not the less complete ; the segments, when three are present, 

 heing wedged hi}o each other in a direction from above downwards, and from 

 without i7iwards ; the first of these movements, by tending to flatten the segments, 

 pressing their margins and sides together : the second, by urging the segments 

 towards the axis of the vessel, impacting them more and more tightly, especially 

 towards their apices or points. As the apices or points formed by the doubling 

 of the segments whilst in action, are composed principally of the flexible and free 

 crescentic margins, and are at liberty to move until the wedging process is com- 

 pleted, a careful examination has satisfied me, that they rotate to a greater or less 

 extent before the valve is finally closed. This spiral movement, which is simply 

 indicated in the venous valves, is more strongly marked in the semilunar ones of 

 the pulmonary artery and aorta (Plate XXVIII. figs. 26, 27, and 28, v, w, oc,) and 

 attains, as will be shown subsequently, a maximum in the auriculo-ventricular 

 valves of the mammal (Plate XXIX. figs. 53 and 54, min,sr). 



By whatever power the blood in the veins advances — whether impelled by the 

 heart alone, or by muscular contractions occurring in different parts of the body, 

 or by rythmic movements which take place in the vessels themselves, or by 

 efforts of inspiration, or by all or combinations of these ; there can, I think, be 

 little doubt that this fluid, in its backward or retrograde movement, acts to a 

 great extent mechanically on the valves as described. It ought, however, to be 

 borne in mind that the veins and the valves are vital structures, and that 



* The angle is never precisely 60°, from the fact of the segments varying slightly as regards size. 

 f The crescentic partitions, as they occur in the semilunar valves of the pulmonary artery and 

 aorta, are shown at 6 b', of fig. 25, Plate XXVIII. 



