The renal vein is removed. A large accessory renal artery entered the
posterior pole. Sections of arteries in limiting layer between
cortical and medullary substance are white in figure.
]
=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.[136]
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.[137]
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=.[138]
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.
[Illustration:
FIG. 379.—TRANSVERSE SECTION OF RIGHT KIDNEY OF HORSE THROUGH THE
HILUS.
Posterior portion of organ hardened _in situ_. Note curvature of
dorsal surface.
]
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=[139]
(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.
[Illustration:
FIG. 380.—DIAGRAMMATIC SCHEME OF URINIFEROUS TUBULES AND BLOOD-VESSELS
OF KIDNEY.
Drawn in part from the descriptions of Golubew (Böhm, Davidoff, and
Huber).
]
=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.