" Thus the Malpighian bodies are as unlike, as the tubes passing from them are like, the membrane which, in other glands, secerns its several characteristic products from the blood. To these bodies, therefore, some other and distinct function is with the highest probabi lity to be attributed.
" When the Malpighian bodies were con sidered merely as convoluted vessels, without any connection with the uriniferous tubes, no other office could be assigned them than that of delaying the blood in its course to the capillaries of the tubes, and the object of this it was impossible to ascertain. Now, however, that it is proved that each one is situated at the remotest extremity of a tube, that the tufts of vessels are a dis tinct system or capillaries inserted into the interior of the tube, surrounded by a capsule formed by its membrane, and closed every where except at the orifice of the tube, it is evident that conjectures on their use may be framed with greater plausibility.
" The peculiar arrangement of the vessels in the Malpighian tufts is clearly designed to produce a retardation in the flow of the blood through them ; and the insertion of the tuft into the extremity of the tube, is a plain indi cation that this delay is subservient in a direct manner to some part of the secretive process.
" It now becomes interesting to inquire, in what respect the secretion of the kidney dif fers from that of all other glands, that so anomalous an apparatus should be appended to its secerning tubes. The difference seems obviously to lie in the quantity of aqueous particles contained in it ; for how peculiar soever to the kidney the proximate principles of the urine may be, they are not more so than those of other glands to the organs which furnish them.
" This abundance of water is apparently in tended to serve chiefly as a menstruum for the proximate principles and salts which this secretion contains, and which, speaking gene rally, are far less soluble than those of any other animal product. This is so true, that it is common for healthy urine to deposit some part of its dissolved contents on cool ing.
" If this view of the share taken by the water be correct, we must suppose that fluid to be separated either at any point of the secreting surface along with the proximate principles, as has hitherto been imagined, or else in such a situation that it may at once freely irrigate the whole extent of the secern ing membrane. Analogy lends no counte nance to the former supposition ; while to the latter, the singular position and all the details of the structure of the Malpighian bodies, give strong credibility.
" It would indeed be difficult to conceive a disposition of parts more calculated to favour the escape of water from the blood than that of the Malpighian body. A large artery breaks up in a very direct manner into a num ber of minute branches, each of which sud denly opens into an assemblage of vessels of far greater aggregate capacity' than itself, and from which there is but one narrow exit. Hence must arise a very abrupt retardation in the velocity of the current of blood. The vessels in which this delay occurs are un covered by any structure.* They lie bare in a cell from which there is but one outlet, the orifice of the tube. This orifice is encircled
by cilia in active motion, directing a current towards the tube. These exquisite organs must not only serve to carry forward the fluid already in the cell, and in which the vascular tuft is bathed, but must tend to remove pres sure from the free surface of the vessels, and so to encourage the escape of their more fluid contents. Why is so wonderful an ap paratus placed at the extremity of each urini ferous tube, if not to furnish water to aid in the separation and solution of the urinous products from the epithelium of the tube?" There is nothing which appears to afford greater support to Mr. Bowman's theory than the structure of the kidney of the boa, when considered in connexion with the fact that the urine in this animal is excreted in an almost solid form. It will be rememberedt that the greater part of the blood supplied to the kid ney of the boa is derived from a vein which comes from the posterior part of the body ; this vein forms the plexus which surrounds the uriniferous tubes, and from which, accord ing to Mr. Bowman, the solids of the urine are excreted. The renal artery, which is com paratively of small size, is distributed to the Malpighian bodies, as in the higher animals, and the efferent vessel joins the portal vein. The solid urine of the serpent seems neces sary consequence of the peculiar distribution of the blood-vessels ; the small Malpighian bodies pour out a scanty stream of water sufficient only to carry through the tubes the large quantities of solid matter which the more numerous and larger vessels distributed on the outer surface of the tubes are continu ally supplying.
Another fact confirmatory of Mr. Bow man's theory has been observed by myself.* In examining the kidneys of persons who had died jaundiced, and in whose urine there had been a large quantity of bile, I observed that the tubes were stained of a deep yellow colour by the bile in their epithelical cells, and that this yellow colour ceased abruptly at the neck of the Malpighian capsule, and in no instance did it affect any part of the tissue of the Malpighian bodies. There are certain other pathological phenomena, which Mr. Bowman's theory very much assists to explain, and which in their turn afford important evi dence in support of the doctrine in question.
The office of secreting the solids of the urine is limited to the convoluted portions of the tubes. The straight tubes of the pyra mids probably have no secreting power, but act merely as excretory ducts to convey the secreted products from the cortical portion of the gland. The different function of these two portions of the tubes is sufficiently mani fested by two facts : — 1st. By the difference in the character of their epithelial lining ; By the fact, that when the cortical por tion of the kidney is the seat of a morbid deposit in consequence of the attempted ex cretion of abnormal products by the epithe lial cells in the convoluted tubes, the medul lary portion of the gland is very commonly free from all trace of the same morbid deposit. This is very frequently obserVed in instances of fatty degeneration, as well as in the earlier stages of the inflammatory diseases of the kidney.