OF THE VALVES OF THE VASCULAR SYSTEM IN VERTEBRATA. 773 



slips (Plate XXVIII. figs. 20 and 29 r), to radiate in a downward and inward 

 direction, i.e. in the direction of the mesial line (Plate XXVIII. fig. 20 c) and body 

 of the segment. These fine slips interdigitate in the mesial line, and are attached 

 below to the uppermost of a series of very strong fibrous bands which occupy 

 the body of the segment (Plate XXVIII. figs. 20 and 29 s). In the interspaces 

 between the slips, the valve is so thin as to be almost transparent. Those portions 

 of the segments included within the delicate fibrous band, running along the free 

 margin and the uppermost of the stronger bands occupying the body, and which 

 are situated to the right and left of the mesial line, are somewhat crescentic in 

 shape (Plate XXVIII. fig. 20 ?•), and have, from this circumstance, been termed 

 lunulse. They do not form the perfect crescents usually represented in books, the 

 horns of the crescents directed towards the mesial line of the segment (Plate 

 XXVIII. fig. 20 c), being much broader than those directed towards the extremi- 

 ties, or where the segments unite above (Plate XXVIII. fig. 20 h). The object 

 of this arrangement is obvious. The crescentic portions referred to, are those which, 

 when the segment is folded upon itself during the action of the valve, are accu- 

 rately applied to corresponding and similar portions of the two remaining seg- 

 ments (Plate XXVIII. fig. 25 h h'). If, however, the lunulse had been symme- 

 trical, in other words, if they had terminated in well-defined horns towards the 

 mesial line, or where the segments fold upon themselves, then the union between 

 the segments in the axis of the vessel (Plate XXVIII. fig. 25 x), where 

 great strength is required, ivoidcl have been very partial, and conseqikntly very 

 imperfect. 



Proceeding from the attached extremities of the segments at points a little 

 below the origins of the marginal band, and curving in a downward and inward di- 

 rection, is the first of the stronger bands (Plate XXVIII. fig. 20 V). The band referred 

 to essentially consists of two portions, these splitting up into brush-like expansions 

 as they approach the mesial line (c), where they interdigitate and become strongly 

 embraced. Other and similar bands, to the extent of three (s) or four, usually the 

 latter number, are met with (Plate XXVIII. fig. 29 -y), and as they all curve in a 

 downward and inward direction, and have finer bands running between them in a 

 nearly vertical direction, they suspend the body of the segment ; so that when 

 water is poured upon it, the various parts of which it is composed, radiate from 

 the attached or convex border like a fan ; each band dragging upon that above it ; 

 the whole deriving support from the thickened convex border. The bands, which 

 are thus six in number, are best seen on that aspect of the segments which is 

 directed towards the sinuses of Valsalva.* They are thickest at their attached 

 extremities, where they interlace slightly, and are mixed up to a greater or less 

 ' extent with the pale, soft, flattened fibres, and elastic tissue of the central layer 



* The surfaces of the segments directed towards the axis are perfectly smooth, and so facilitate 

 the onward flow of the blood. 



VOL. XXIII. PART in. 9 X 



