OF THE VALVES OF THE VASCULAR SYSTEM IN VERTEBRATA. 775 



In other cases (Plate XXVIII. fig. 21), the marginal band not only splits up and 

 contains a fully developed Corpus Arantii {d)^ but gives off two or more well- 

 marked tendinous slips {a) which connect the free margin with the stronger or 

 central portion of the segment {i). Radiating from the Corpus Arantii as a 

 centre (Plate XXVIII. fig. 24 d), and proceeding along the free margin, I have 

 sometimes detected a series of hair-like fibres [e), which are apparently of use 

 in strengthening this the weakest portion of the segment. This is the more 

 probable, since other and similarly delicate fibres proceed from the attached 

 extremities in the direction of the Corpus Arantii. On other occasions, the 

 tendinous bands proceeding from the marginal one (Plate XXVIII. fig. 22 s) are 

 abnormally thickened {t), and terminate in brush-shaped expansions in the body 

 of the segment {v) ; the body under such circumstances, projecting in an upward 

 direction towards the Corpus Arantii {d). In such cases, those portions of the 

 valve {r r') which occur between the thickened bands proceeding from the mar- 

 ginal one, are exceedingly thin, and in some diseased conditions altogether awant- 

 ing, so that the segment very much resembles one of the segments of the nutral 

 or tricuspid valve, with its chordae tendinese. That there is an analogy between 

 the semilunar and the mitral and tricuspid valves, and that the chordae tendinese 

 is a further development, seems probable from the fact, that in the bulbus 

 arteriosus of certain fishes, as in the grey and basking sharks (Plate XXIX. 

 figs. 41 and 48), Lepidosteus, &c. (Plate XXIX. fig. 40 h), the semilunar valves 

 are furnished with what may be regarded as rudimentary chordae tendinese, 

 (Plate XXIX., fig. 48. «), while in the auriculo-ventricular valves of fishes, which 

 have hitherto been regarded as semilunar, but which exhibit some of the pecu- 

 liarities of the mitral valve of the mammal, chordae tendineae in various stages 

 of development occur. 



A scheme of the arrangement of the tendinous bands in the semilunar valves 

 has been given at Plate XXVIII. fig. 23, and shows the segments to be not only 

 bilaterally symmetrical, but to be constructed on a plan which secures the greatest 

 amount of strength with the least possible material ; the bands mutually acting 

 upon and supporting each other. Thus the bands marked a and d, which re- 

 present the central portion of the segment, split up into brush-shaped expansions, 

 one portion of each curving in an upward direction (h e), and representing the 

 tendinous slips proceeding from the marginal one {r) ; the remaining portions 

 curving in a downward direction (/c), and giving the inferior set of fibres which 

 curve from below, towards the body of the segment {ad). The Corpus Arantii 

 is rarely present in a perfectly healthy semilunar segment ; nor will its absence 

 occasion surprise, when it is remembered that its presence materially interferes 

 with the folding of the segments upon themselves, when the valve is in action. 

 That its existence is not necessary to the perfect closure of the valve, is proved by 

 its complete absence in a great number of cases. In the semilunar valve of the 



