Fig. 1. Diagram of the uriniferous tubule.

R, Cortical substance.

Gr, Zone of limitation.

M, Medullary substance.

empties in connection with similar tubes into the conducting tubes, which, uniting with many other similar tubes, form the comparatively large ducts which discharge their contents into the calices of the renal pelvis. The circulation of the kidney is peculiar. The renal artery, after reaching the pelvis, divides into numerous branches, one of which is distributed to each lob

ule, and again breaks up into numerous branches, which form arches at the junction of the cortical and medullary portion, and a small arterial twig, the afferent vessel, enters each one of the capsules of Bowman, at a point just opposite the neck already spoken of. It is there divided into numerous branches or loops, the Malpighian tuft, which reunite and finally makes its exit at the same point

pighi

2, Convoluted tubes (Tubuli Contorti).

3a, Henle's loops, descending or small branch;

vessels then break up into

b. ascending or large branch; c, intermediate a true capillary system,

piece.

4, Excretory tubes, (tubes of Bellini); d, junc- which ramify over the tutional tubes; e, collecting tubes of the first class;

f, collecting tubes, second class.

G, Papillary orifice.

buli uriniferi, particularly

Malpighin

3

the convoluted tubes, and finally unite to form the initial branches of the renal vein. See Fig., after Gray's Anatomy. The urinary tubules are lined throughout with epithelium, which varies in character in the different portions.That of the convoluted portion, according to the researches of Heidenhain, is of a peculiar rod-like character and cloudy appearance which encroaches considerably upon the lumen of the tubule, as you will see in the figure which Kidneys (Gray's Anatomy). accompanies, copied from him.* The tubule at the descending portion of Henle's loop, beside becoming very much constricted in caliber, undergoes a profound modification in the character of its epithelium; it being clear and tessilated, swolen slightly at the nucleus and strongly resembles that which lines the blood-vessels. Indeed, it is difficult to distinguish between a transverse section of the two under the microscope. The ascending portion of Henle's loop is lined by an epithelium, which very closely resembles the rodlike epithelium which lines the convoluted portion. The so-called intermediate piece is lined by a flattened cylindrical epithelium. In the conduction tubes of the first class, the tube with rod-like epi- epithelium is flattened, clear, and the lumen of the tube relatively large. The larger collecting tubes are lined by a modified cylindrical epithelium. This constriction of the ascending limb of Henle's loop is of importance, in the study of tube casts, as we will see further on when we come to study their significance and morphological importance. The secretion of the urine, the exact meth*Charcot, op. cit.

Fig. 3. Convoluted

od of its accomplishment, and the physiological function of the kidney, has been very carefully studied by many competent observers, and yet it cannot be said that all the questions connected therewith have been satisfactorily and unequivocally settled.

It is universally agreed, however, that the separation of the urine is effected in the cortical portion exclusively. One of the questions to decide has been whether the kidneys are true glandular organs, elaborating substances of a characteristic nature, or simply strainers separating these substances from the plasma of the blood in which they exist already formed. Such observers as Hoppe-Seyler, Oppler and Zaleskey, have thought that the formation of the urea, uric acid, &c., took place within the kidneys, probably by the further oxydation of the nitrogenous waste substances. The question whether there is a greater amount of urea in the renal artery or vein by the observations of Grehart appears to have been settled in the affirmative. This difference disappears after ligating the ureters. According to some observers (Bernard, Barreswill, Oppler and Zalesky,) after exterpation of the kidneys or ligature of the renal vessels, no accumulation of urea takes place in the blood; while Meissner, Voit and Grehart, maintain that such accumulations do take place. If the former account be correct, it is still no evidence of the formation of urea in the kidneys; for, according to the observations of Bernard and Barreswill, animals so operated on excrete vicariously from the mucous membrane of the stomach and bowels, fluids containing urea and ammonia. But the animals so quickly succumb from uræmia, whatever that may be, that there is no time for a large accumulation of urea. The further oxidations of creatine and creatinine and their conversion into urea has been suggested. Creatine and creatinine, however, when administered, as a rule, appear unaltered in the urine and not as urea (Meissner, Voit). * Hermann concludes, that in the present

* Human Physiology, 1875.

state of our knowledge, we must incline to the belief that urea and uric acid are separated from the blood and not found in the kidneys, and suggests that the liver is the principal seat for the production of these two important urinary constituents. For of all organs it yields most urea or (in case of birds) uric acid (Hensius, Stokvis), and yields urea to blood which is passed through its vessels. Hermann rather leans to the theory advanced by Ludwig as to the method by which the separation is effected, viz: that resistance to the blood through the vessels of the Malpighian tuft, by reason of the secondary or true capillary system, causes a free filtration of water from the blood into the capsule and uriniferous tubule, together with those substances (salts, sugar, urea, &c.,) which form true solutions. Albumen, &c., which do not form true solutions, do not filter through except under conditions. of abnormally increased pressure.

"The very dilute solution thus formed, comes into proximity through the walls of the tubuli uriniferi, with the blood which it has just left, and which is in a concentrated state owing to the loss of water; diffusion must result, leading to a return of the water into the blood, and to a consequent concentration of the urine." Hermann, however, further says: "Besides these physical processes, other causes seem to co-operate in the formation of urine." "Many circumstances lead us to suppose that the glandular cells (epithelium) especially assist; pathological degeneration of the epithelial cells, for example, interferes with secretion; and in birds uric acid deposits are seen to originate within the cells, the disintegration of which seems necessary before the deposits can become free (Wittich, Meissner). This is the theory advanced by Bowman, who thought that a peculiar affinity on the part of these cells for the small quantities of the urea, uric acid, &c., contained in the blood, and their constant separation in this way was sufficient to account for the amount present in the urine. The researches of Heidenhain, who, injecting a weak solution of indigo-sodique into the

blood of an animal, found that a blue color was quickly communicated to the kidneys and urine, while a like discoloration was communicated to no other part of the organism. He calls attention to the very close analogy between the phenomena here exhibited and the elimination of urea and uric acid, specific principles of the urine. If, as has been asserted by Ludwig, the whole of the elements secreted in the urine are separated at the same time as the water, and at the same points in the uriniferous tubules, the suppression of the secretion of the water would necessarily involve the suppression of the specific constituents. On injecting the indigosodique in an animal, when the spinal cord has been divided high up, not a drop of urine was secreted, or reached the bladder; the blue tint, however, was communicated to the kidney, showing that it had been secreted, if not excreted. On section, however, it was found not generally diffused throughout the kidney substance as is the case when the secretion of the watery portion is not interfered with, but limited to the cortical portion. Microscopic examination shows that it is only particular portions of the organ that exhibit the blue tint, that have been charged with the elimination, viz: the convoluted portion and the ascending portion of Henle's loops, the capsules of Bowman, and the collecting tubes show not the slightest trace of color. The canaliculi contorti and the ascending portion of the loop are thus shown to possess independent functions, if not absolute and positive power of secretion, as claimed by Bowman.

Their peculiar cloudy rod-like epithelium recall the arrangement of other secreting organs. Furthermore, if the animal be killed ten minutes after the injection, the blue color impregnates the epithelial cells alone, but if killed at the end of one hour the cells are found to be clear, the coloring matter has passed into the tubes where, in the absence of water, it is found in a very concentrated state.

It may be asked, but does this prove that the salts and other matter peculiar to the urine, are eliminated in this way.