Mr. Bowman thus draws a comparison be tween the circulation through the kidney of the Boa and that through the liver " The circulation through this form of kidney may be aptly compared with that through the liver, as described by Mr. Kiernan in his invaluable paper on that gland. The plexus surrounding the tubes corresponds with the portal-hepatic plexus, which, in the lobules of the liver,invests the terminal portions of the bile-ducts. Both these plexuses are supplied with blood by a portal vein, derived chiefly from the capillaries of distant organs, but in part from those of the artery of the respective organs them selves. The only difference seems to be, that, while in the liver the branches of the artery are entirely given to the larger blood vessels, ducts, &e., in the kidney a few only are so distributed, the greater number go ing through the Malpighian bodies, to per form an important and peculiar function. In both glands, however, all the blood of the artery eventually joins that of the portal vein. The emulgent vein of the kidney answers to the hepatic vein of the liver." " The comparison between the hepatic and the renal portal circulation may be thus drawn in more general terms. The portal system of the liver has a double source, one extraneous, the other in the organ itself ; so the portal system of the kidney, in the lower tribes, has a two-fold origin, one extraneous, the other in the organ itself. In both cases the extra. mous source is the principal one, and the artery furnishing the internal source is very small. But in the kidney of the higher tribes the portal system has only one internal source, and the artery supplying it is proportionably large." Mr. Bowman has ascertained that in all the vertebrate classes the Malpighian bodies have essentially the same structure ; the capsule being formed by the dilated extremity of a uriniferous tube, into which a single mass of blood-vessels is inserted. But in some orders of animals there are modifications which merit notice. The most considerable of these re gard the size of the Malpighian bodies. The following table from Mr. Bowman's paper ex hibits their size in a few species, and subjoined to each measurement is that of the tube soon after its emergence. It will be seen that the diameter of the tubes varies far less than that of the Malpighian bodies.
According to Professor Miller* the kid ney of the rnyxinoid fishes has a very simple structure. Before the publication of Mr. Bowman's paper Miiller described the kidney of these fishes, as consisting of a long ureter extending on each side of the intestine, and sending off at intervals a small sac which terminates in a second closed sac, the junction of the two sacs being marked by a constric tion. In the cavity of the closed sac there is a globular tuft of vessels, which is free on all sides except at one point, where the vessels pierce the investing capsule (fig. 162.). Prof. Miiller, from a comparison of his own obser vations with those of Mr. Bowman, infers that the short tubes proceeding from the ureter in these fishes are analogous to the uriniferous tubes in the more highly orgaaised kidneys, while the closed sac at the extremity of the tube is analogous to the Malpighian capsule ; so that each rcnculus in the myxinoid fish consists of an exceedingly short uriniferous tube terminating in a capsule, in which is sus pended a globular tuft of vessels. The arte
rial branches which come directly from the aorta terrninate, as in the higher animals, by piercing the capsule and forming a globular tuft within it. Miller had not an opportu nity of tracing the exact distribution of the blood after leaving the capsule, but he thinks it probable that the veins form a plexus on the outer surface of the tubes. It is to be regretted that Miiller has not given some account of the microscopic appearances pre sented by the inner surface of these tubes, since without some observations on this point, and particularly with reference to the charac ter of the epithelium, it is not possible to form a definite notion as to the exact nature of the parts in question.
Epitheliuni. — In exarnining the epithelium of the kidney, it will be convenient to com mence with that of the Malpighian bodies, and thence to trace this structure through the tubes into the pelvis and ureter. It is scarcely possible' to overestimate the importance of a c'reful study of the epithelial cells in different parts of the kidney, since accurate observations upon this point must form the basis of an exact knowledge of the physiology 'of the gland, and of the pathological changes to which it is liable.
Epithelium of the Malpighian bodies.— With reference to the epithelium of the Malpighian bodies, it will suffice to recapitulate here what has already been fully detailed in speaking of the structure of these bodies. The epithelium of the Malpighian bodies consists of two dis tinct portions : first, that which covers the vessels ; and, secondly', that which lines the capsule. The vessels of the Malpighian tuft appear to have in many instances a more or less complete investment of small, delicate, and transparent nucleated cells. (Fig. 160.) These cells differ entirely from those on the inner surface of the capsule, as well as from those which line the urinary tubes. The epi thelium covering that part of the capsule which is contiguous to the orifice of the tube is very transparent, and clothed with vibratile cilia. This ciliated epithelium covers about one-third of the inner surface of the capsule beyond this point the cilia cease, and the epithelium is of excessive delicacy and translucence (figs. 158. and 159.), while in many instances it is impossible to detect the slightest appearance of epithelium beyond the line where the cilia cease. The cilia in this situation have been observed only in reptiles and fishes, but they probably exist in all classes of Vertebrata.
Epithelium of the uriniferous tubes. — The epithelium of the uriniferous tubes presents itself in two distinct forms, the one kind exist ing in the convoluted tubes of the cortex, and the other in the straight tubes of the medullary cones. The epithelium in that part of the uriniferous tubes immediately continuous with the Malpighian capsule, presents the same characters as that which covers the contiguous portion of the capsule, consisting of delicate transparent particles, which in fishes and rep tiles are furnished with vibratile cilia.