tral vessel. This arrangement gives to the vessels of the fibrous envelope of the kidney a peculiar stellate appearance. These are sometimes called the stars of Verheyen. The large trunks which form the centres of these stars then pass through the cortical substance to the rounded bases of the pyramids, where they form a vaulted venous plexus corresponding to the arterial plexus already described. The vessels distributed upon the straight tubes of the pyramidal substance form a loose plexus around these tubes, except at the papillæ, where the net-work is much closer. They then pass into the plexus at the bases of the pyramids to join with the veins from the cortical substance. From this plexus a number of larger trunks arise and pass toward the hilum in the centre of the inter-pyramidal substance, enveloped in the same sheath with the arteries. Passing thus to the pelvis of the kidney, the veins converge into from three to four great branches, which unite to form the renal, or emulgent vein. A preparation of all the vessels of the kidneys shows that the veins are much more voluminous than the arteries.1

The lymphatics of the kidney are few, and, according to Sappey, only exist in the substance of the organ, converging toward the hilum. This author does not. admit the existence of superficial lymphatics.

The nerves are quite numerous, and are derived from the solar plexus, their filaments following the artery in its distribution in the interior of the organ and ramifying upon the walls of the vessels.

1 In a recent pamphlet on a circulation peculiar to the kidney of mammals, a French author assumes to have demonstrated an arrangement of bloodvessels in the cortical substance very different from that which we have described. The glandular character of the Malpighian bodies and their connection with the convoluted tubes are denied. There is apparently so little basis for these peculiar views, that it does not seem necessary to discuss them in detail, and we will simply refer the reader to the original monograph. (SUCQUET, D'une circulation du sang spéciale au rein des animaux vertébrés mammifères, et de la sécrétion des urines qu'elle y produit, Paris, 1867.)

Summary of the Physiological Anatomy of the Kidney. -The division of the kidneys into the cortical and pyramidal substance is quite apparent to the naked eye. The pyramids are distinctly striated, and present, in this regard, and in their darker color, a marked difference from the cortical substance. At the apex of each pyramid there are from two hundred to five hundred little orifices, from to of an inch in diameter, which connect with the straight tubes. From these openings the tubes branch at a very acute angle, each one leading to a bundle or system of straight canals, forming the collections called the pyramids of Ferrein. The branches of these tubes (the tubes of Bellini) are about of an inch in diameter, and are composed of a structureless membrane lined by nucleated epithelial cells.

When these tubes arrive at the bases of the pyramids and pass into the cortical substance, they increase slightly in size, and are lined with granular and rounded cells of epithelium. They then become excessively convoluted, connect with certain other tubes in their course, and after forming looplike processes extending into the pyramids, finally terminate in rounded or ovoid dilatations (the Malpighian bodies). These dilated extremities measure from to of an inch in diameter.

The Malpighian bodies are composed of a fibrous capsule (the capsule of Müller), and each one contains a mass of convoluted blood-vessels surrounded by nucleated epithelial cells.

The loop-like processes dip down into the pyramids and return to the cortical substance, present a filamentous, constricted portion, and are here called the narrow tubes of Henle. The communicating tubes, which connect these canals with the straight tubes of the pyramidal substance, are sometimes called "intermediate tubes." They are flattened or ribbon-shaped, with very delicate walls, and are lined by transparent pavement-epithelium.

Throughout the kidney there is a delicate stroma of fibrous tissue, in the meshes of which are lodged the blood

vessels, the straight tubes of the pyramidal substance, and the tubes and Malpighian bodies of the cortical substance.

The renal artery penetrates the kidney at the hilum, sends branches between the pyramids, which are distributed in the form of an arched arterial plexus over the upper portion and the bases of the pyramids, following exactly the boundary between the pyramidal and the cortical substance. From these vessels, branches are given off both on the convexity and the concavity of the arches. Numerous small branches (arteriola recta) pass downward along the straight tubes toward the papillæ, becoming capillary as they surround the tubes. Other branches take an opposite direction and pass into the cortical substance, breaking up into little twigs, each one of which penetrates a capsule of Müller and divides in its interior into a mass of looped, convoluted vessels which constitute the Malpighian coil. The blood is carried away from the Malpighian bodies by one, two, or three vessels, which are then immediately distributed in a close plexus around the tubes of the cortical substance. From this plexus, the radicles of the renal vein pass to the surface of the kidney, where they present a stellate arrangement, converging toward several large central vessels (the stars of Verheyen). These central vessels penetrate the cortical substance and form an arched venous plexus over the rounded bases of the pyramids. This plexus also receives by its concave surface venous branches from the pyramidal substance. The blood is then emptied into larger veins, passing between the pyramids in the same sheath with the arteries, to form the renal or emulgent vein.

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CHAPTER VI.

MECHANISM OF THE FORMATION AND DISCHARGE OF URINE.

Formation of the excrementitious constituents of the urine in the tissues, absorption of these principles by the blood, and separation of them from the blood by the kidneys-Effects of removal of both kidneys from a liv. ing animal-Effects of tying the ureters in a living animal-Extirpation of one kidney-Influence of blood-pressure, the nervous system, etc., upon the secretion of urine-Effects of the destruction of all the nerves going to the kidneys-Alternation in the action of the kidneys upon the two sides-Changes in the composition of the blood in passing through the kidneys-Physiological anatomy of the urinary passages-Mechanism of the discharge of urine.

THE striking peculiarities which the kidney presents in its structure, as compared with the true glands, and the fact of the voluntary discharge of its secretion at certain intervals, would naturally lead to a closer study of the mechanism of the production and discharge of the urine, than we have given under the general head of mechanism of the formation of the excretions. The composition of the urine, also, will be found to be exceedingly complex, and its various ingredients bear the closest relation to the processes of nutrition and disassimilation; all of which considerations render it of the greatest importance to ascertain the precise mode of its formation, and to study all the conditions by which this process may be modified. In the present state of our knowledge, we must certainly regard the excrementitious constituents of the urine as formed essentially in the system at large, and merely separated from the blood by the kidneys; and a consideration of these effete principles belongs to the

subject of nutrition. It remains for us, then, in this connection, to treat, in general terms, of the way in which these substances find their way into the urine.

The most important constituent of the urine is urea; a crystallizable nitrogenized substance, which is discharged by the skin as well as by the kidneys. This has long been recognized as an excrementitious principle; but the first observations that gave any definite idea of the mechanism of its production were made by Prévost and Dumas,' in 1821. At the time these experiments were made, chemists were not able to detect urea in the normal blood; but Prévost and Dumas extirpated the kidneys from living animals (dogs and cats), and found an abundance of urea in the blood, after certain symptoms of blood-poisoning had been manifested. The first experiments were performed by removing one kidney by an incision in the lumbar region, and at the end of three or four days, after the animal had recovered from the first operation, removing the other. After the second operation the animals lived for from five to nine days. For the first two or three days there were no symptoms of blood-poisoning. Watery discharges from the stomach and intestinal canal occurred after a few days, and finally stupor and other marked evidences of nervous disturbance supervened, when the presence of urea in the blood could be easily determined. These observations were confirmed and extended by Ségalas and Vauquelin, in 1822, who presented to the French Academy of Medicine a specimen of nitrate of urea extracted from the blood of a dog, taken sixty hours after extirpation of the kidneys, giving its proportion to the weight of blood employed. A few years later, the observations of Prévost and Dumas were con

The observations of Prévost and Dumas, Ségalas, Marchand, and others, have already been referred to (see p. 25).

SÉGALAS, Sur des nouvelles expériences relatives aux propriétés médicamenteuses de l'urée, et sur le gendre de mort qui produit la noix vomique.-Journal de physiologie, Paris, 1822, tome ii., p. 356.