The manner in which the tubes actually terminate is by becoming continuous with the Malpighian bodies. This fact, which can be demonstrated in many of the tubes, is a matter of fair inference and of moral certainty in the case of every tube. The proofs of this fact, and the precise mode of continuity, we shall presently proceed to examine.
Structure of the uriniferous tubes contain the two structures which ally compose the mucous tissue, viz. the basement nzembrane and the epitheiium.
The basement membrane is a thin transpa rent homogeneous lamina, simple and entire, without any aperture or appearance of struc ture. It forms the parenchymal wall of the uriniferous tubes ; gives them their form, size, and stability ; is in relation, on the one hand, with the vascular system of the organ and the investing fibrous matrix, and on the other, with the epithelial lining of the tubes. The epithelium adheres to the inner surface of the membrane by organic union : it sometimes separates readily after maceration in water, and in some forms of chronic inflammation of the kidney it ,frequently happens that the epi thelial lining of many of the tubes is entirely removed, or only a few particles of epithelium remain scattered over the inner surface of the membrane. " It sometimes happens, that when the epithelium may seem to be alto gether detached, the basetnent membrane re tains, scattered evenly over its surface and at some distance apart, a number of roundish marks, of the size and aspect of the nuclei of epithelium particles. These are most probably the early condition of the new or advancing series of these particles."# The basement membrane is united externally to the capillary venous plexus and the investing fibrous matrix ; there is probably some organic con nexion between these tissues, which allows of the free transudation of materials from the blood-vessels through the basement membrane to the epithelial cells. When a tube deprived of its epithelium is detached from the sur rounding tissue, the basement membrane is readily thrown into folds and wrinkles, and appears to possess a considerable amount of elasticity. The thickness of this membrane, according to Mr. Bowman, does not exceed --,47–th of an English inch. In certain I. 0 diseased states of the kidney its thickness is much increased, and simultaneously the cavity of the tube becomes dilated so as greatly to exceed its normal diameter, thus constitutina the serous cysts which are so frequently ob6 served in the kidney.
The basement membrane of the tubes is continuous on the one hand with the capsule of the Malpighian bodies, and on the other through the straight tubes of the pyramids with the basement layer of the mucous membrane which lines the pelvis of the kidney.
The mean diarneter of the tubes is about ,44. inch. The entire diameter of the convo luted tubes in the cortical portion somewhat exceeds that of the straight tubes in the pyra mids, although the cavity of the latter is greater than that of the former. The latter fact resulting, as will be seen hereafter, from the difference in the character of the epithe lium in these portions of the tubes.
After the brief allusion just now made to the epithelium of the tube and its relation to the basement membrane, it will be more con venient to postpone for the present the par ticular consideration of this important struc ture, and to proceed to the examination of the Malpighian bodies. We shall then revert to the epithelium, and we shall find that the varying characters which it presents in different parts of the organ are of the great est interest and importance in connection with the physiology of the renal secretion, as well as on account of the assistance which they all'ord in the interpretation of the patho logical changes to which the kidney is liable.
Malpighian The Malpighian bo dies have been objects of much interest since their discovery by the distinguished anatomist whose name they bear. Malpighif ascertained that these bodies, which he calls internal glands, could be readily injected from the arteries, to the branches of which they are appended. He could not succeed in in jecting them from the veins, in consequence, as he believed, of valves in these vessels pre venting the passage of the injected material in a backward direction ; he, however, considers it a rational inference, that the venous radi cles commence in these bodies. Malpighi further endeavoured to demonstrate the con nection which he believed to exist between these bodies and the urinary tubes. He made unsuccessful attempts to inject the tubes from the arteries and from the veins : and, finally, he experimented on a living dog, by tying the renal veins and the ureters. On examination of the kidneys after death, there were some appearances of the renal glandules (the Mal pighian bodies) and the tubes being con nected and continuous ; but he confesses that his opinion on this point was derived rather from analogy than from ocular demonstration, his idea being that the urinary constituents were separated from the arteries of the Mal pighian bodies, and that the tubes were the excretory ducts of these glands.