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Show to the portal theory of the renal cardinal mesh work-a very necessary preliminary for the suggestions to follow. Theoretical Objections to the Portal Theory of the Renal Card inal Meshwork. Attributing a portal function to the renal cardinal meshwork ordinarily implies that the kidney substance eliminates from the venous blood circulating through its sinuses a (nitrogenous) product which is excretory * in nature. It also follows from this supposition that those animals which have a " renal-portal system " possess a, considerable advantage over animals which have not, since obviously in the former a far larger proportion of the blood passes through the kidney (the " portal " kidney being " supplied '* from both an arterial and a venous source), and hence there exist greater facilities for excretion. Assuming then the hypothesis to be valid, we may reasonably expect to discover that a " renal-portal system '* exists in all those animals which manifest the greatest activity, since, activity being coextensive with the amount of excretory matter produced, they need it most. Organs in general are best developed, i. e. most efficient in their mode of function, in those animals where they are most required, and an excretory renal-portal system being undeniably an improvement on the non-portal kidney, this should conform to the general rule. The expectation however is not justified by the facts : on the contrary, we find that in general the more active the animal the less developed is the " renal-portal system " ! In the relatively inactive cold-blooded Pisces, Amphibia and Reptilia, the " renal-portal system " attains its maximum of development, whereas in the active hot-blooded Aves and Mammalia it is either reduced or absent. And this same irrelation between activity of the animal and the presence of the renal cardinal meshwork can also be observed in more special cases : e. g.r the caudal vein forms a conspicuous renal meshwork in Pleuronectidae, but does not form one at all in the Perch and Salmon ; indeed the degree to which the renal cardinal meshwork is developed in Teleosts (and other groups) is extremely variable. Thus, apart from other considerations to be adduced, it follows solely from the general rule just mentioned that the renal cardinal meshwork is probably not portal in function. Evidence of a more special character tends to the same conclusion. The kidney, like all other organs, conforms to another general law, viz., that size is proportional to work done. It is well known for example that excision of one of the kidneys causes the companion organ to enlarge considerably owing to the additional activity thus deputed to it, and it is equally well known that the most active animals possess the largest kidneys relatively to the size of the * The following arguments equalljr appl}r on the supposition that the product is a secretion (like that of the thyroid gland, e.g.) which is subsequently added to the blood. Puoc. Zool. Soc.-1906, No. LIX. 59 1 9 0 0 .] " JlENAL-POllTAL SYSTEM.'' 8 8 7 |