68 



MU. D. K. BHATTACHARYA ON THE 



the ligament. Fig. 7 (Plate II.) represents diagram maticalty the 

 exact disposition of the ligament in the anterior trunk-region and 

 beneath the basioccipital bone. Tracing the ligament in the 

 trunk-region from the posterior to the anterior end, we find that 

 behind the region of the third vertebra the ligament is well deve- 

 loped and hangs deep down into the cavity of the aorta (Plate 11. 

 fig. 14). In the region of the third vertebra the ligament 

 does not hang so deep down, but gradually ascends towards the 

 dorsal wall of the aorta, and consequently the suspensory fold 

 becomes narrower dorso-ventrally, though thicker from side to 

 side (Plate II. fig. 13). Just beneath the region where the second 

 vertebra ends, the ligament enters the wall of the aorta (Plate II. 

 fig. 12). Here, in the outer dorsal wall of the aorta (in the 

 adventitia layer), a thick mass of cartilaginous tissue is developed 

 (Plate II. fig. 11, c), which, as I have already said, acts like a pad 

 or cushion, and is always situated close behind where the ligament 

 is attached to the bone. Figs. 9, 10, and 11 (Plate II,) show the 

 ligament running in the wall of the aorta and gradually ascending 

 till we find that the ligament actually perforates the dorsal wall 

 of the aorta and becomes attached to the bone (Plate II. figs. 7 

 and 8, lig.per.). A branch, however, runs through the wall of the 

 aorta and pierces through its most anterior region (fig. 7, lig.a.') 

 close to the place whei-e the posterior efferent branchial vessels meet. 

 It then runs close beneath the basioccipital and is finally attached 

 to the vomer as in Pseudeutroj^ms garua. In Sihmdia gangetica 

 ( Plate I. fig. 5) the ligament pierces through the dorsal wall of 

 the aorta a short distance behind its origin. The major portion 

 of the ligament becomes attached to the basioccipittd bone, and 

 a branch runs forwards beneath the basioccipital and the para- 

 sphenoid. Towards its anterior end the ligament breaks up in a 

 fan-like manner and forms a thin sheet of elastic and connective 

 tissue (fig. 5, «.). Posteriorly the ligament in all cases ends 

 either in the last vertebra or in the fused urostyle and hypural 

 bone. 



lY. The Occurrence and Absence of the Ligament in various 



Fishes. 



The aortic ligament is by no means a sti'ucture of universal 

 occurrence. It is confined to some of the Teleost fishes alone, 

 being totally absent from the Cyclostomes, Elasmobranchs, 

 Amphibians, Reptiles, Birds, and Mammals. In the Teleostomes 

 it seems to be more of a generic feature than of a family one. The 

 tabular statement (pp. 70-72) shows that, whereas it is present 

 in the various species of a genus, it is absent from other genera of 

 the same family. I have adopted here Goodrich's classification 

 chiefly, as given in Lankester s ' A Treatise on Zoology,' but I 

 have also followed in some places Day's system of classification 

 (' Fauna of British India, Fishes,' vols. i. and ii.). 



The ligament is present in all the Clupeidse and Cyprinidse that 

 I have examined, but in the large family of Siluridas it seems to 



