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Show 1906.] " RENAL-PORTAL SYSTEM." 895 Mammals. A discussion concerning the renal cardinal meshwork woidd be incomplete without a reference to the conspicuous non-u portal condition of the kidney found in mammals. The mesonephros of mammalian embryology forms a renal cardinal meshwork identical with that found in lower vertebrata, but with the development of the mammalian metanephros, with its extreme concentration of the kidney substance and a-median position (being situated externally to the transverse processes of the vertebrae), the kidney of the adult mammal loses all connection with the post-caval vein from the " portal " standpoint and is solely supplied with blood by the renal artery *. The two features just noted as characteristic of the mammalian kidney probably account for the absence of the renal cardinal meshwork. Concentration of the kidney substance means a grouping together of the kidney tubules largely in the dorso-ventral direction, and a corresponding shortening in longitudinal extension; in other words, the area of attachment of the kidney becomes more restricted, and hence there must necessarily be less tendency to encroach upon the venous channels. Indeed, in the higher mammals, the kidneys are only prevented from sharing the transposition of the testes (1 1 ) by means of a " circumambient development of areolar tissue which usually contains much fat," and is especially developed in connection with this tendency to displacement t. That is to say, besides having no adjacent posterior cardinals upon which to encroach, the kidneys in the higher mammals are, as it were, shaken clear of all surrounding structures. As Owen remarks, the mammalian kidneys " have a more compact and definite form than in birds, and their vascular system is more exclusively their own " J. * It is evidently not true, as Johnson (3) alleged, that the kidney of Mammalia, possessing no " portal " system, has a proportionately lai'ge artery to supply the supposed deficiency. f " Maintenance of the position of the kidney is chiefly dependent upon the integrity of the connective-tissue investment............ If we take those cases in which the kidney was lowest down, we find chest diseases prominent accompaniments." Dr. C. Addison in Proc. Anat. Soc. Gt. Brit. & Ireland, May, 1904. I should like to add here in connection with my paper on the phylogenetic cause of the transposition of the mammalian testes, published in 1903, that, despite its crudity in several parts, I am not aware that a better idea on the subject has since been suggested. I certainly cannot credit Dr. Keith's hypothesis, e. g. (‘ Human Embryology and Morphology,' 2nd ed. p. 156), because, to supply two obvious reasons only, intratesticular pressure is probably as great as intraabdominal pressure, and the testes in the lower mammals are not shut off from the abdominal cavity and in the active birds do not descend at all. X This and the above statements supply a full answer to the question raised by Minot (" On the Veins of the Wolffian Bodies in the Pig," in Proc. Boston Soc. Nat. Hist. vol. x. 1898), as to the possible phylogenetic significance of the difference obtaining between the sinus-like character of the channels permeating the mesonephros and the true capillarisation of the " metanephros of the higher vertebrata. As Minot says, " in the [mammalian] metanephros theintertubular vessels are true capillaries and not sinuses," and the arguments I have above adduced show why this should be so. There is no breach of genetic continuity as Minot suggests- the non-portal " metanephros " is not a " new " organ, but simply a posterior development of a mesonephros which has not encroached upon a venous sinus. |