The =spleen= (Lien) is the largest of the ductless glands.[102] It is
situated chiefly in the left parachondrium, in close relation to the
left part of the great curvature of the stomach, to which its long axis
corresponds. Its size and weight vary greatly in different subjects, and
also in the same subject under different conditions, depending chiefly
on the great variability of the amount of blood contained in it. The
average weight is about 35 ounces (ca. 1 kg.), its length about 20
inches (ca. 50 cm.), and its greatest width about 8 to 10 inches (ca. 20
to 25 cm.). It is usually bluish-red or somewhat purple in color. In the
natural state it is soft and yielding, but not friable.
The weight appears to vary ordinarily from about one to eight pounds,
although in large horses the latter figure even may be exceeded
without any apparent evidence of disease. There does not seem to be
any constant relation to the body-weight. For example, the spleen of a
colt about ten months old weighed three and a half pounds, while it
often weighs less than two pounds in horses weighing 1000 to 1200
pounds. The chief variation in outline consists of increase of width,
especially of the dorsal part.
It extends obliquely in a curved direction from the left crus of the
diaphragm and the saccus cæcus of the stomach to the ventral third of
the ninth or tenth rib. It presents for description two surfaces, two
borders, and two extremities.
The =parietal= or =external surface= (Facies parietalis) is convex, and
lies chiefly against the diaphragm, but is in direct contact with the
upper parts of the last two ribs and to a small extent with the flank at
the lumbo-costal angle.
The =visceral= or =internal surface= (Facies visceralis) is in general
concave. It is divided into two unequal portions by a longitudinal
ridge; on this is a groove, the =hilus=, in which the vessels and nerves
are situated. The area in front of the ridge (Facies gastrica) is
moulded on the great curvature of the stomach; it is about two inches
(ca. 5 cm.) wide. The area behind the ridge (Facies intestinalis) is
much more extensive; it is related chiefly to the small colon, the small
intestine, and the great omentum. It may be marked by one or two
fissures.
[Illustration:
FIG. 277.—SPLEEN OF HORSE, HARDENED _in situ_. VISCERAL SURFACE.
The area marked intestinal impression is related to the first coil of
the small colon.
]
The =anterior border= (Margo anterior) is concave and thin.
The =posterior border= (Margo posterior) is convex and thin.
The =base= or =dorsal extremity= is beveled, and fits into the interval
between the left kidney and the left crus of the diaphragm and the psoas
major. When hardened _in situ_, it shows an impression (Facies renalis)
where it lies against the kidney. The left extremity of the pancreas
touches it also. The anterior basal angle fits in between the saccus
cæcus of the stomach and the left kidney; the posterior basal angle
usually lies against the flank just behind the last rib.
The =apex= or =ventral extremity= is small; it lies—when the stomach is
not full—between the left lobe of the liver and the left dorsal portion
of the colon; when the stomach is full, the spleen is pushed further
back and loses contact with the liver.
The spleen is attached by two peritoneal folds, the suspensory ligament
and the gastro-splenic omentum. The =suspensory ligament= (Lig.
suspensorium lienis) attaches the base to the left crus of the diaphragm
and the left kidney; it contains a quantity of elastic tissue. The part
which passes to the diaphragm is the ligamentum phrenico-lienale, and
blends with the gastro-phrenic ligament; the part which goes to the
kidney is termed the ligamentum renolienale. The =gastro-splenic
omentum= (Lig. gastrolienale) passes from the hilus to the left part of
the great curvature of the stomach. It is narrow above, where it joins
the suspensory ligament; below it becomes much wider and is continuous
with the great omentum.
Small globular or lenticular masses of splenic tissue may be found in
the gastro-splenic omentum. They are termed =accessory spleens=
(Lienes accessoriæ).
=Structure.=—The spleen has an almost complete =serous coat=. Subjacent
to this and intimately united with it is a =capsule= of fibrous tissue
(Tunica albuginea), which contains many elastic fibers and some
unstriped muscular tissue. Numerous =trabeculæ= (Trabeculæ lienis) are
given off from the deep face of the capsule and ramify in the substance
of the organ to form a supporting network. In the interstices of this
framework is the =spleen pulp= (Pulpa lienis), a dark red, soft, grumous
material. This is supported by a delicate adenoid =reticulum=, and
contains numerous leukocytes, the large splenic cells, red
blood-corpuscles, and pigment. The pulp is richly supplied with blood.
The branches of the splenic artery enter at the hilus and pass along the
trabeculæ. The arteries which enter the pulp have a sheath of lymphoid
tissue, which collects on the vessel wall at certain points, forming
small =lymph nodules=, the so-called Malpighian corpuscles (Noduli
lymphatici lienales). These are visible to the naked eye as white spots,
about as large as the head of a pin. The blood passes into cavernous
spaces lined by endothelium which is continuous with the cells of the
reticulum of the pulp. From these the veins arise. The splenic vein runs
in the hilus in company with the artery and nerves, and joins the
posterior gastric vein to form a large radicle of the portal vein.
=Blood-supply.=—Splenic artery.
=Nerve-supply.=—Splenic plexus.