1906.] " RENAL-PORTAL SYSTEM." 895 



Mainifiials. 



A discussion concerning the renal cardinal meshwork would be 

 incomplete without a reference to the conspicuous non-" portal" 

 condition of the kidney found in mammals. The mesonephros 

 of mammalian embryology forms a renal cardinal meshwork 

 identical with that found in lower A'ertebrata, but with the 

 develojament 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 

 vertebrse), 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. Con- 

 centration of the kidney substance means a groujaing together of 

 the kidney tubules laigely in the dorso-venti'al direction, and a 

 corresponding shortening in longitudinal extension ; in other 

 words, the ai-ea 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 (11) 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 f. That is to say, besides having no adjacent 

 postei*ior cai-dinals 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 kidne^ys " have a 

 more compact and definite form than in birds, an.d their vascular 

 system is more exclusively their own " J. 



* It is evideutlj' not true, as Johnson (3) alleged, that the kidnej' of Mammalia, 

 possessing no " portal " sj'stem, has a propoi'tionately large artery to supply the 

 supposed deficiency. 



t " 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. 1 should like 

 to add here in connection with my paper on the phylogeuetic cause of the trans- 

 i:)Osition 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 sujDply two obvious 

 reasons only, intratesticular pressure is probablj' as great as intraabdominal pressure, 

 and the testes in the lower mammals are not shut otf from the abdominal cavitj'' 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 meso- 

 nephros and the true capillarisation of the " metanephros " of the higher vertebrata. 

 As Minot says, " in the [mammalian J metanephros the intertubular 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. 



