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Show 1906.] u RENAL-PORTAL SYSTEM." 897 1 .lood from the hind-limbs and tail in the mid-dorsal channel, but no full explanation has yet been offered to account for the fact that in the majority of the Ichthyopsida and Sauropsida, part of the venous blood derived from the aforenamed regions adopts a mid-ventral course in its passage to the heart. What is the explanation of the production of pelvic veins ? We cannot suppose that the blood from the limbs is thus in part deviated from the primitive mid-dorsal course merely in order to pass into the portal system of the liver, since the absence of any such feature in most Fishes and in all Mammalia effectually negatives any such idea*. Owen indeed elaborated an ingenuous though somewhat fragile hypothesis which supported this idea, but, as he himself admitted, it was of little value and solely appertained to the coccygeo-mesenteric of birds and not indeed always to that f . In my opinion, a possible solution to this problem lies in the fact that the adoption of a mid-ventral way of return by a large proportion of the venous blood is in all cases (save, and for the reason below stated, Chelonia, and perhaps Apteryx and other Ratitfe, in which there exists little or no meshwork) correlated * Owen remarks in connection with this point that £:the anastomosis of the pelvic veins, in being the means of conveying common venous blood into the liver, goes to prove that the blood of the vena) porta) does not require any peculiar preparation by circulation in the spleen or other viscera to tit it for the secretion of bile." If my theory is correct, it would seem that this is no proof. t Since this ingenuous theory of Owen is, so far as I know, the only one hitherto propounded to account for the presence of epigastric veins in Vertebrata, it seems worth while quoting him fully on the subject. " The venous system of the kidneys is so arranged in birds that the blood can be distributed either to the portal system by the mesenteric vein, or to the pulmonary system by the vena cava and right side of the heart, according to the degree of rapidity with which the pulmonary or portal systems of veins are respectively supplied, or in other words, according to the activity with which the circulation in each of these systems may be going on at two different periods............. This disposition has been erroneously supposed to indicate that the urine was secreted from the venous blood in birds, as in reptiles and fishes ; but the end attained by the venous anastomoses in question bears a much closer relation to the peculiar necessities and habit of life of the bird, and, so far as I know has not hitherto been explained. There is no class of animals in which there may be, at any two brief and consecutive periods of existence, a greater difference in the degree of energy and rapidity with which the respiratory functions are performed than in birds. When the bird of prey, for example, stimulated by a hungry and an empty stomach, soars aloft and sweeps the air in quest of food, the muscular energies are then strained to the utmost, the heart beats with the most forcible and rapid contractions to propel the current of blood along the systemic arteries, and the pulmonary vessels require the greatest possible supply of blood to serve the heart with the due quantity of arterialised fluid : the digestive system, on the other hand, is in a state of repose and we may conceive the portal circulation to be at its lowest ebb. Suppose the eagle to be glutted with his quarry and reduced to a state of torpor ; the animal functions are now at rest, but the organic powers concerned in the assimilation of the food are in full play, and the portal or hepatic circulation is as active as was the pulmonary a short time before." But since the same disposition of veins occurs in Apteryx, Owen admits that " the modifications of this part of the venous system were less important than I had been led to anticipate in a bird whose comparatively limited powers of locomotion must be attended with less partial and excessive action of the respiratory system than in birds of flight " (‘‘ On the Anatomj' of the Southern Apteryx," Trans. Zool. Soc. vol. ii. 1841). Moreover, the theory obviously does not apply to the similar venous arrangement found in Cercitodus, Amphibia, and Reptilia. In mammals, on the other hand, which most resemble birds in the alternating activity of the respiratory and portal systems, a communication between the portal and post-renal veins does not exist! |
