The Urinary Organs of the Horse

THE KIDNEYS

Each kidney (Ren) presents two surfaces, two borders, and two extremities or poles, but they differ so much in form and position as to require a separate description of each in these respects.

The right kidney (Ren dexter) in outline resembles the heart on a playing card, or an equilateral triangle with the angles rounded off. It lies ventral to the upper parts of the last two or three ribs and the tip of the first lumbar transverse process. The dorsal (or superior) surface (Facies dorsalis) is strongly convex; it is related chiefly to the diaphragm, but also to a small extent posteriorly to the psoas muscles. In well hardened specimens, especially those from thin subjects, impressions of the last two ribs and the tip of the first lumbar transverse process are usually visible. The ventral (or inferior) surface (Facies ventralis) is in general slightly concave, and is related to the liver, pancreas, cæcum, and right adrenal; it either has no peritoneal covering, or only a narrow area externally is so covered. The internal border (Margo medialis) is convex and rounded; it is related to the right adrenal and the posterior vena cava. It presents about its middle a deep notch, the hilus (Hilus renalis); this is bounded by rounded margins, and leads into a space termed the renal sinus (Sinus renalis). The vessels and nerves reach the kidney at the hilus, and the sinus contains the pelvis or dilated origin of the ureter. The external border (Margo lateralis) is rounded, and is thinner than the internal one. It consists of two parts, anterior and posterior, which meet at an external angle; the anterior part fits into the renal impression of the liver. The duodenum curves around the external border. The anterior extremity (Extremitas cranialis), thick and rounded, lies in the renal impression of the liver. The posterior extremity (Extremitas caudalis) is thinner and narrower.

The left kidney (Ren sinister) is bean-shaped. It is considerably longer and narrower than the right one, and is situated nearer the median plane and further back, so that the hilus of the left kidney is about opposite to the posterior extremity of the right one. It lies usually under the upper part of the last rib and the first two or three lumbar transverse processes. The dorsal surface is convex, and is related to the left crus of the diaphragm, the psoas muscles, and the base of the spleen. The ventral surface is convex and irregular; the greater part of it is covered by the peritoneum. It is in relation with the origin of the small colon, the terminal part of the duodenum, the left adrenal, and the left extremity of the pancreas. The internal border is longer, straighter, and thicker than that of the right kidney. It is related to the posterior aorta, the adrenal, and the ureter. The external border is related chiefly to the base of the spleen. The anterior extremity extends almost to the saccus cæcus of the stomach; it is related to the left end of the pancreas, and the splenic vessels. The posterior extremity is usually larger than the anterior one.

The form of the left kidney is variable. In some cases its outline is similar to that of the right kidney, but its ventral surface is convex and is often marked by several furrows which diverge from the hilus. In well hardened specimens the three areas of the dorsal surface are often distinct. The psoas area is flat, parallel with the inner border, and widens behind. The diaphragmatic area is small and convex; it is crescentic and is confined to the anterior pole. The splenic area, convex and external, is often so extensive and distinct as really to constitute a third surface.

Fixation.—The kidneys are held in position chiefly by the pressure of adjacent organs and by the renal fascia. The latter is a special development of the subperitoneal tissue, which splits into two layers to inclose the kidney, together with the perirenal fat or capsula adiposa. On account of its relations with the liver, pancreas, and the base of the cæcum, the right kidney is much more strongly attached than the left one. It is, therefore, not surprising that the latter varies somewhat in position; its posterior pole may be found ventral to the third or fourth lumbar transverse process. The position of the right kidney seems to be very constant, excluding its movements during respiration.

Weight and Size.—The right kidney is usually one or two ounces heavier than the left. The average weight of the right kidney is about 23 to 24 ounces (ca. 700 gm.); that of the left, about 22 to 23 ounces (ca. 670 gm.). The relation of the weight of both kidneys to the body-weight is about 1 ∶ 300–350.

Chauveau gives as an average 750 grams for the right kidney, and 710 grams for the left. Ellenberger and Baum (24 cases) give the right kidney as varying between 430 and 840 grams; and the left between 425 and 780; this is an average of 635 grams (about 22½ ounces) for the right kidney, and 602.5 grams (about 21½ ounces) for the left. They give the relation of the weight of both kidneys to the body-weight as 1 ∶ 255–344. In a Percheron mare weighing about 2000 pounds the right kidney weighed 4 pounds 3 ounces, and the left one 4 pounds. In a horse of medium size the right kidney is about six inches (ca. 15 cm.) in length, about the same in width, and about two inches (ca. 5 cm.) thick. The left kidney is about seven inches (ca. 18 cm.) long, four to five inches (ca. 10 to 12 cm.) wide, and two to two and a half inches (ca. 5 to 6 cm.) thick.

Structure.—The surface of the kidney is covered by a thin but strong fibrous capsule (Tunica fibrosa), which is in general easily stripped off the healthy kidney. It is continued along the hilus and lines the renal sinus. Sections through the kidney show it to consist of an external cortical substance and an internal medullary substance. The cortical substance (Substantia corticalis) is red brown in color and has a granular appearance. It is dotted over with minute dark points; these are the renal (or Malpighian) corpuscles (Corpuscula renis), each consisting of the dilated origin of a uriniferous tubule (Capsula glomeruli), with an invaginated tuft of capillaries (Glomerulus) inclosed by it. The medullary substance (Substantia medullaris) is more resistant and presents a distinct radial striation. Its central part is pale, but its periphery, the intermediate zone, is of a deep red color; in the latter are seen, at fairly regular intervals, sections of the relatively large arciform vessels, which are taken to represent the demarcation between the primitive lobes. Between the vessels the medulla is prolonged somewhat toward the periphery, forming the bases of the renal (or Malpighian) pyramids. These are not very pronounced in the kidney of the horse, especially as the gland is not papillated. Between the bases of the pyramids processes of the cortex dip in toward the sinus, forming the renal columns. The inner central part of the medulla forms a concave ridge or crest which projects into the pelvis of the kidney; the crest is marked by numerous small openings at which the renal tubules terminate.

Examination with a pocket lens shows that the cortex is imperfectly divided into lobules (Lobuli corticales). Each lobule consists of an axial radiate part (Pars radiata), surrounded by a convoluted part (Pars convoluta). The former (formerly termed pyramids of Ferrein) appear as ray-like prolongations from the bases of the pyramids (hence also termed medullary rays), and consist largely of narrow, straight or slightly flexuous, tubules (limbs of the loops of Henle). The convoluted part (formerly termed the labyrinth) is granular in appearance, and consists largely of the renal corpuscles and convoluted tubules.

The pelvis of the kidney (Pelvis renalis) is the dilated origin of the excretory duct. It lies in the sinus of the kidney, and it is funnel-shaped, but flattened dorso-ventrally. The renal crest (Papilla communis) projects into the outer part of the pelvis in the form of a horizontal ridge with a concave free edge. The tubules of the middle part of the medullary substance open on this crest into the pelvis. The tubules from each end of the kidney do not open into the pelvis proper (Recessus medius), but into two long, narrow diverticula (Recessus terminales), which proceed from it toward the poles of the kidney. The wall of the pelvis consists of three layers. The external fibrous coat or adventitia is continuous with the supporting tissue of the kidney. The muscular coat consists of longitudinal and circular fibers. The mucous coat does not cover the renal crest, nor is it continued into the diverticula of the pelvis. It has a yellowish tinge, and presents numerous folds. It contains glands (Glandulæ pelvis renalis) which secrete the thick mucus always found in the pelvis.

Renal Tubules—The parenchyma or proper substance of the kidney is composed of the small renal or uriniferous tubules (Tubuli renales), which are very close together and have a complicated course. Each tubule begins in a thin-walled, spherical dilatation or capsule, which is invaginated to receive a tuft of looped capillaries termed a glomerulus; these two structures constitute a renal (or Malpighian) corpuscle; the corpuscles are visible as minute red or dark spots in the convoluted part of the cortex. Succeeding this is a short narrow neck, beyond which the tubule becomes wide and convoluted, forming the proximal convoluted tubule, and enters the radiate portion of the cortex. It then gradually narrows and enters the intermediate zone; becoming very narrow and nearly straight, it descends for a variable distance into the medullary substance, turns sharply upon itself, and returns to the cortex, forming thus the loop of Henle, with its descending and ascending limbs. In the convoluted part of the cortex, it widens and becomes tortuous, constituting the distal convoluted tubule. The tubule then narrows, enters a medullary ray, and opens with other tubules into a straight collecting tubule; this passes axially through a pyramid, and unites with other collecting tubules to form the relatively large papillary ducts, which open into the renal pelvis.

Stroma.—The interstitial tissue forms a reticulum throughout which supports the tubules and blood-vessels. It is very scanty in the cortex, much more abundant in the medulla, in which it increases in amount toward the pelvis.

Blood-supply.—The kidneys receive a large amount of blood through the renal arteries. Branches of these enter at the hilus and on the ventral surface of the gland, and reach the intermediate zone, where they form anastomotic arches (Arteriæ arciformes). From these arciform arteries branches pass into the cortex and medulla. The cortical branches (Arteriæ interlobulares) have in general a radial course between the cortical lobules, and give off short lateral branches, each of which ends as the afferent vessel (Vas afferens) of a renal corpuscle. The blood is carried from the glomerulus by a smaller efferent vessel, which breaks up immediately into capillaries which form networks around the tubules. The medullary branches descend in the pyramids, forming in them bundles of straight twigs (Arteriolæ rectæ).

The veins correspond in general to the arteries. In the superficial part of the cortex the veins form star-like figures (Venæ stellatæ) by the convergence of several small radicles to a common trunk.

Lymph Vessels.—These may be divided into two sets, capsular or superficial, and parenchymatous or deep.

Nerve-supply.—The nerves are derived from the renal plexus of the sympathetic, and form a plexus around the vessels.