AORTIC LIGAMENT IN INDIAN FISHES. 



69 



be absent from a few genera, though present and well-developed 

 in the majority. 



V. Some Sit'jgestions concerning the Bole and Belationslii'ps 

 of the Aortic Ligament. 



If a fish possessing an aortic ligament ])e completely stripped 

 of its muscles, the vei'tebral column (with its dorsal elastic 

 ligament and the attached aoi'ta) becomes thrown into a series of 

 serpentine curves. If now the aortic ligament be removed, the 

 curves disappear. Tliis is proof (1) that the lateral muscles 

 keep the aortic ligament on the stretch ; (2) that the aortic 

 ligament, which is not closely adherent along its course to the 

 vertebral column, is antagonistic in action to the dorsal elastic 

 ligament, the latter tending to keep straight the vertebral 

 column, the former tending to curve it ; (3) that the aortic 

 ligament is more powerful than the dorsal elastic ligament ; 

 (4) that when the vertebral column becomes flexed by muscular 

 action, the dorsal elastic ligament becomes stretched and the 

 aortic ligament slackened. These obvious conclusions, howevei-, 

 do not assist us much in comprehending the function of the 

 aortic ligament. What this function can be, it is at present 

 only possible to guess at. It would seem that it must be difierent 

 to that of Reissner's fibre in the nerve-cord and of the dorsal 

 elastic ligament, if only because of its diff'erent situation and 

 attachments. It would also appear that, in view of the fact that 

 the aorta in these, as in other fishes, assumes the functions of 

 a heart (the actual heart being separated from the aorta by the 

 gill capillary system), the aortic ligament must either act as an 

 auriculo- ventricular valve or actively assist in the propulsion of 

 blood along the aorta. Since it is difficult to conceive in what 

 way any alteration of position or tension of the vertical curtain 

 of tissue formed by the ligament and its suspensory fold in the 

 aorta can enforce the contractions of the aortic wall, the only 

 feasible explanation of the aortic ligament is to suppose that it 

 acts as a longitudinal valve preventing forward regurgitation of 

 blood. According to Burne, Stewart suggested that since the 

 aortic ligament, owing to its stoutness and tension, remains 

 straight during the lateral flexions of the body, it, with its 

 suspensory fold, must act as a diagonal curtain which sweeps the 

 blood posteriorly in the aorta during the swinnning movements 

 of the fish. The flexions of the body originating anteriorly and 

 passing posteriorly, successive portions of the aortic ligament 

 curtain will assume a dias^onal position in the aorta as the flexion 

 proceeds posteriorly, and this process, it was suggested, may aid 

 in the propulsion of the blood posteriorly. Since, however, the 

 fold hangs loosely in the aorta and never completely closes it, 

 it seems more likely that this diagonal curtain will he of more 

 use as a valve which to some extent prevents regurgitation of the 

 blood forwards, than as a mechanism which would have to be of 



