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



of the vessel itself. They in this manner form a fibrous zone (Plate XXVIII. fig. 

 17 57^), which corresponds to the attached convex border of the segment, and 

 may be regarded as an expansion of the inner of the two divisions (Plate XXVIII. 

 fig. 17 tr) into which, as I formerly pointed out, the pulmonary artery and aorta 

 resolve themselves. 



As the bands under consideration are exceedingly strong when compared with 

 those occurring in other portions of the segment; and project in an inward direc- 

 tion, or towards the axis of the vessel, when the preparation is sunk in water ; 

 their function, as ascertained from numerous experiments on the semilunar 

 valves of a whale {Physalus antiquorum, Gray), seems to be the following : — 



\st, They carry the body of the segment away from the sides of the vessel, and 

 incline the free margins towards each other, at such an angle, as necessitates the free 

 margins of neighbouring segments, being always more or less in apposition. In this 

 they are assisted by the thickened portion of the pulmonary artery (Plate XXVIII. 

 figs. 17 and 36 b n) which projects between the segments (Plate XXVIII. fig. 36 

 n n') where they unite above, and by the fibrous zones which correspond to the 

 convex border of each segment. 



2d, The stronger fibres suspend the body of the segment from above, and permit 

 the reflux of blood to act more immediately upon the mesial line of each segment where 

 thinnest (Plate XXVIII. fig. 20 c), and where least supported ; to occasion that 

 characteristic folding of the segment upon itself, when the valve is in action. 



Other bands, intermediate in thickness between those occupying the free 

 margin and the body of the segment, are found towards its lower portion (Plate 

 XXVIII. figs. 20 and 29 o). These bands cross and interdigitate to a greater or 

 less extent, and as their prevailing direction is from below upwards, are instru- 

 mental in keeping the lower poi^ions of the segment, away from the sides of the 

 vessel. Each segment may therefore be described as consisting of three por- 

 tions — a superior and thinner portion, an inferior and thicker one, and a central 

 portion, which is the thickest of all. It ought also to be remarked that the three 

 portions of the segment corresponding to its mesial line, where the folding occurs 

 when it is in action, are comparatively thinner than the parts to either side of 

 the line in question. The varying thickness of the segments is well seen in the 

 semilunar valve of the whale (Plate XXVIII. fig. 2d av n). 



While the foregoing may be considered a literal description of a healthy 

 human semilunar valve, there are modifications to which it is necessary to direct 

 attention. Thus, in some instances, the band occupying the margin of the segment 

 splits up near its origin, as represented at Plate XXVIII. fig. Id b, and maps out a 

 triangular portion (c). This portion is very thin, and contains delicate tendinous 

 fibres, which terminate in brush-shaped expansions towards the mesial line. 

 The remaining and stronger bands are similar to those already described, but the 

 difference in the thickness of the several portions of the valve, is not so marked. 



