. Transactions of the Royal Society of New Zealand . ems to me that an argument distinctly in favour of this theory, isafforded by the case now under consideration. The ancestral vertebratepossessing the continuous lateral fin must presumably have had a largelateral vein, into which opened numerous veinlets from the fin, and it isreasonable to suppose that, as certain portions of the fin developed at theexpense of the rest, the anterior and posterior ends of the vein, in relationwith them, would take on a greatly increased size, the intermediate partbecoming proportionately reduced, and supply
. Transactions of the Royal Society of New Zealand . ems to me that an argument distinctly in favour of this theory, isafforded by the case now under consideration. The ancestral vertebratepossessing the continuous lateral fin must presumably have had a largelateral vein, into which opened numerous veinlets from the fin, and it isreasonable to suppose that, as certain portions of the fin developed at theexpense of the rest, the anterior and posterior ends of the vein, in relationwith them, would take on a greatly increased size, the intermediate partbecoming proportionately reduced, and supplying finally, only the body-wall between the fore and hind-limbs. According to Professor St. George Mivart, the skate presents us with anearer approach than any known type, to the primitive form of limb-skeleton—the archipterygium. If this view be correct it is very interestingto find the limb possessing what we may consider as an almost primitivevenous supply. But it must be remembered that, the skate being in many TRANS. NZINSUTUTUOLMPffi . VENOUS SYSTEM of /MM /VASVfA ad no., Parker.—On the Venous System of the Skate. 417 respects an extremely specialized fish, certainly far more specialized thanthe sharks or dog-fish, it may be that the primitive arrangements of itslimb-skeleton and veins should be looked upon as re-acquired, rather thanas retained. There is one general point about the skates venous system to which Imay, in conclusion, direct attention, and that is the extraordinary numberof transverse anastomoses it presents, the result being to produce numer-ous venous circles, comparable to the circle of Willis in the arteries ofthe mammalian brain, and the circuius cephalicus in the arterial system ofbony fishes. Thus there is a direct passage from the sinus venosus andback again, in four different ways, namely: (1) by the the hepatic sinus ; (2)by the anterior part of the cardinal vein and the cardinal sinus ; (3) by thewhole length of the cardinal veins
Size: 1187px × 2105px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1800, bookdecade1880, booksubjectscience, bookyear1880
