The =adrenal bodies= or =suprarenal glands= (Glandulæ suprarenales)[142]
are two small, flattened organs, which lie in contact with the anterior
part of the inner border of the kidneys (Figs. 376, 377). They are
ductless.
In the horse they are red-brown in color, about three and a half to four
inches (ca. 9 to 10 cm.) long, one to one and a half inches (ca. 3 to 4
cm.) wide, and about half an inch or more (ca. 1 to 5 cm.) in thickness.
The weight varies from one to two ounces (ca. 28 to 56 gm.).
The =right adrenal= is related internally to the posterior vena cava, to
which it is adherent. Its anterior part curves dorsally around the inner
border of the right kidney. Its posterior part is flattened and is
related ventrally to the pancreas and cæcum, dorsally, to the right
renal vessels. The anterior extremity is concealed in the renal
impression of the liver; the posterior is related to the ureter.
The =left adrenal= is a little shorter than the right one, and its
extremities are rounded; it is usually curved, so that its internal
border partly embraces the anterior mesenteric artery. Its dorsal
surface is related to the kidney, the renal artery, the aorta, and the
left cœliaco-mesenteric ganglion. The ventral surface is in relation
with the left extremity of the pancreas and the root of the great
mesentery. The posterior extremity often curves inward behind the
anterior mesenteric artery; it is related behind to the left renal vein.
When hardened _in situ_ the adrenals present several features not
evident in the soft organs. The anterior part of the right adrenal is
twisted upward and outward over the inner margin of the kidney, so
that this part is prismatic and has three surfaces; of these, the
concave outer one is applied to the kidney, the dorsal one is related
to the right crus of the diaphragm and the liver, and the inner one is
in contact with the posterior vena cava. The anterior mesenteric
artery is nearly always more or less enlarged as a result of verminous
arteritis, and the form and degree of curvature of the left adrenal
seem to vary in conformity with the condition of the artery.
=Structure.=—The =fibrous capsule= adheres intimately to the surface of
the organ. It contains elastic fibers, and in its deep part unstriped
muscle-fibers. From it trabeculæ pass radially into the substance,
blending with the fine supporting reticulum. The =parenchyma= consists
of a cortical and a medullary portion. The =cortical substance=
(Substantia corticalis) is red-brown in color, and is clearly
distinguishable from the yellow =medullary substance= (Substantia
medullaris). A large =central vein= (Vena centralis) is visible on
cross-sections.
The cells of the cortex are arranged in chains of one or two rows. In
the peripheral portion the cells are of high cylindrical shape, and
the chains form connecting loops; Günther has proposed the name zona
arcuata for this part, instead of the usual term zona glomerularis.
More deeply the chains are distinctly palisade-like, and this region
is called the zona fasciculata. Next to the medulla is the zona
reticularis, in which the chains form a network. The cells in these
two zones are polygonal and contain a brown pigment.
The cells of the medulla are arranged in irregular groups or form
sheaths around the veins. They react to chromic salts by assuming a
yellow or yellow-brown color, and are termed chromaffin cells; they
share this peculiarity with certain cells of the sympathetic ganglia
and paraganglia, with which they are probably related genetically. The
alkaloid adrenalin appears to be formed in the medullary cells.
=Vessels and Nerves.=—The adrenals receive a relatively large
blood-supply through the =adrenal arteries= (Aa. suprarenales), which
arise from the renal arteries or from the aorta directly. The =veins=
terminate in the posterior vena cava and the left renal vein. The =lymph
vessels= go to the renal lymph glands. The numerous =nerves= are derived
from the sympathetic system through the solar and renal plexuses. The
fibers form a rich interlacement, especially in the medullary substance.
Ganglion cells are found chiefly in the medulla, but also occur in the
deeper part of the cortex.
[Illustration: