OCR Text |
Show 1906.] ANATOMY OF THE OPHIDIA. 41 example three other veins of the same kind, of which two received branches from the gut; there was also an independent gastro-hepatic vessel, as is shown in the accompanying figure (text-fig. 11, p. 40), which represents the liver-veins of this example. The epigastric vein sends, at any rate, two branches to the liver, which are very anterior in position: one of these receives a branch from the stomach before entering the liver. In another example there were only three dorsal parieto-hepatics. It was in this example that a branch from the epigastric joined the umbilical vein, as referred to above. In a third specimen, I saw only two parieto-hepatic vessels, arising, as in the others, from the left side of the dorsal median line. In this and other cases the differences may not be real, but due to absence of blood in the vessels at the time of examination. The origin of the hinder mesenteric vein in the Ophidia has been variously stated, the different modes of origin described possibly corresponding to the different species and genera examined. Hochstetter * describes and figures Tropidonotus natrix (with which he finds Coluber cesculapii to agree) as possessing a mesenteric vein which arises from both afferent renals, the two branches combining to form the single vein. I have not been able to ascertain to m y satisfaction the arrangement of these veins in Bitis nasicornis; but in another Yiper, Ancistrodon piscivorus, I have found that each renal afferent vein gives off a branch, and that these join to form the mesenteric vein running along the lower surface of the large intestine. The arrangement characteristic of this Viper is therefore precisely that of Tropidonotus and Coluber. (5) Considerations respecting the Primitive Structure of the Lungs in the Squamata. Hatteria (or, indeed, most Lacertilians) on the one hand, and such a snake as Causus rhombeatus on the other, represent the two extremes of modification of the Squamate lung. In the former the lungs are paired and equal, and are effective breathing-organs throughout: they are separated from the glottis by a long stretch of trachea, and by two equisized bronchi into which the trachea divides some way in front of the lungs. In the Yiper, on the other hand, the trachea opens into the lung but a short way behind the glottis, down which it is continued as an open gutter; at, or about, the level of the heart the lung becomes anangious and is a mere air-sac; while there is no trace of a second lung, or of a division of the tracheal gutter into two bronchial tubes. It is undoubtedly the prevalent opinion that of these two extremes, that represented by Hatteria is near to the primitive Sauropsidan lung, while the lung of Causus represents the most modified type. Paradoxical though it will appear, there are reasons founded upon anatomical * Morph. Jahrb. xix. 1893, p. 489, pi. xvi. fig. 19. |