The other system of capillaries, or that surrounding the uriniferous tubes, corre sponds, in every important respect, with that investing the secreting canals of other glands. Its vessels anastomose with the utmost free dom on every side, and lie on the deep sur fitce of the membrane that furnishes the secretion.
Mr. Bowman has applied the term "portal system of the kidney" to the series of vessels connecting these two, on account of the close analogy it seems to bear to the vena porta, intervening, like it, between two capillary net works, the first of which answers to that in which the vena porta originates, and the second to that in which the vena porta ter minates. The capillary plexus surrounding the tubes differs, therefore, from that of other glands, and agrees with that of the liver, in its receiving blood that has previously tra versed another system of capillary vessels.
The correctness of the analogy which Mr. Bowman has drawn between the circulation of the kidney and that of the liver is very beautifully shown by his observations on the kidney of the boa-constrictor, an animal which may be regarded as the type of those in which, besides the renal artery, the kidney receives a portal vein derived from the hinder part of the body.* Mr. Bowman thus de scribes the organ in question :—" The kidney of the .boa, being composed of isolated lobes of a compressed reniform shape, displays all the points of its structure in peculiar sim plicity and beauty. At what may be termed the hilum of each lobe, the branches of the vena porta and duct separate from those of the renal artery and emulgent vein ; the two former spreading side by side, in a fan-like form, over the opposite surfaces of the lobe, while the two latter enter its substance and radiate together in a plane midway between these surfaces. The lobe is made up of the ramifications of these four sets of vessels, in the following mode (fig. 161.). Each duct, as it runs over the surface, sends down a series of branches which penetrate in a pretty direct manner tovtards the central plane. Arrived there, they curl back, and take a more or less retrograde course towards the surface, and, finally, becoming more convoluted, terminate in the Malpighian bodies, which are all situated in a layer at some distance within the lobe, parallel to the central plane, and nearer to it than to the surface. The ducts never anastomose. The artery subdivides into ex
tremely minute twigs, no larger than capil laries, which' diverge on either hand and enter the Malpighian bodies. The efferent vessels are of the same size as the afferent, and, on emerging, take a direct course to the surface of the lobe, and join the branches of the vena porta there spread out. The branches of the portal vein on the surface send inwards a very numerous series of twigs of nearly uniform capacity, and only a little larger than the vessels of the capillary plexus, in which they almost immediately terminate. This is the plexus surrounding the uriniferous tubes. It extends from the surface to the central plane of the lobe, and there ends in the branches of the emu/gent vein." " Thus the efferent vessels of the Malpighian bodies are radicles of the portal vein, and, through the portal vein, empty themselves, as in the higher tribes, into the plexus surround ing the uriniferous tubes. The only real difference between this form of kidney and that of Mammalia is that there is here a vessel bringing blood that has already passed through the capillaries of distant parts, to be added to that coming from the Malpighian bodies, and to circulate with it through the plexus surrounding the tubes. The efferent vessels of the Malpighian bodies run up to the surface, in order to throw their blood through the whole extent of the capillary plexus; which they would fail to do if they entered it in any other part." " I have described the renal artery as being spent upon the Malpighian bodies ; but in the hilum of the lobe it gives off, as in the higher animals, a few slender twigs to the coats of the excretory ducts, and of the larger vessels. The capillaries of these twigs are easily seen, and, in all probability, dis charge themselves into the branches of the portal vein." It will appear on referring to the plan (fig. 161.), that there is a direct relation between the size of the Malpighian bodies and the width of the lobe. At the apex of the lobe, where the uriniferous tubes are com paratively short, the Malpighian bodies are of small size, while at the base of the lobe, where the tubes are longer, the Malpighian bodies present a corresponding increase of size. It will presently be seen that this and other facts in the anatomy of this form of kidney, afford very important evidence as to the nature and office of the Malpighian bodies.