Specimen from middle-aged subject, hardened _in situ_.
]
5. The =left lateral ligament= (Lig. triangulare sinistrum) attaches the
dorsal edge of the left lobe to the tendinous center of the diaphragm.
6. The =hepato-renal= or =caudate ligament= (Lig. hepatorenale) attaches
the caudate process to the right kidney and the base of the cæcum. The
gastro-hepatic omentum and the mesoduodenum have been described.
As stated above, the liver is divided by fissures into three principal
lobes—right, middle, and left. The right lobe is the largest in the
young subject and is irregularly quadrilateral in form. On its dorsal
part is the caudate lobe, which ends in a pointed process directed
outward, and assists in forming the cavity for the right kidney. The
middle lobe is normally much the smallest. The left lobe is oval in
outline and thickest centrally. In old or middle-aged subjects it often
exceeds the right one in size. In some cases the atrophy of the right
lobe is so extreme that the middle lobe may exceed it in size.[99]
The =hepatic duct= (Ductus hepaticus) is formed at the ventral part of
the portal fissure by the union of right and left chief lobar ducts. It
is two or three inches (ca. 5 to 8 cm.) long and about half an inch (ca.
1 to 1.5 cm.) wide. It passes between the two layers of the lesser
omentum, and pierces the wall of the duodenum about five or six inches
(ca. 12 to 15 cm.) from the pylorus, alongside of the pancreatic duct.
The ducts pass obliquely through the wall of the duodenum for about half
an inch (ca. 1 cm.) before opening into the diverticulum duodeni. The
arrangement forms an effective valve, which prevents regurgitation from
the intestine. There is no gall-bladder.
[Illustration:
FIG. 276.—LIVER OF NEW-BORN FOAL, HARDENED _in situ_. VISCERAL
SURFACE.
The differences, when compared with the organ in the adult, are very
striking.
]
=Structure.=—The liver is covered by an outer serous, and an inner
fibrous coat. The =serous coat= covers the gland except at the
attachment of the pancreas and at the portal fossa; it is reflected from
it to form the ligaments and the lesser omentum. The =fibrous capsule=
is in general thin; it sends laminæ into the ligaments, and also
delicate strands into the gland substance. At the portal fissure it is
abundant and surrounds the vessels and ducts, which it accompanies in
the portal canals of the gland substance.
The gland substance is composed of the parenchyma and the interstitial
tissue. The =parenchyma= is made up of lobules, 1 to 2 mm. in diameter,
which are held together by a small amount of interlobular connective
tissue. On account of the very small amount of the latter, the
lobulation of the horse’s liver is not usually at all distinct to the
naked eye; for the same reason the organ is also quite friable.[100]
=Vessels and Nerves.=—The =portal vein= enters at the portal fissure. It
conveys blood from the digestive tract and the spleen, which contains
various products of digestion and numerous white blood-cells. The
=hepatic artery= also enters at the portal fissure; it may be termed the
nutrient vessel. All the blood is returned from the liver to the
posterior vena cava by the hepatic veins. The portal vein and the
hepatic artery both divide into interlobular branches, which run
together in the portal canals of the interlobular tissue. The branches
of the portal vein (Venæ interlobulares) give off intralobular branches
which form plexuses of capillaries in the lobules and give rise to a
central vein (Vena centralis). The branches of the hepatic artery (Rami
arteriosi interlobulares) are of relatively small size. They supply
mainly (if not exclusively) the interlobular tissue, the capsule, and
the walls of the vessels and ducts. The =hepatic veins=[101] (Venæ
hepaticæ) empty into the vena cava as it lies in the fossa of the gland.
Their ultimate radicles are the central lobular veins, which emerge from
the bases of the lobules and join the sublobular veins (Venæ
sublobulares); the latter unite to form the hepatic veins. The largest
hepatic veins, three or four in number, join the posterior vena cava
just before it leaves the liver to pass through the diaphragm.
The nerve-supply comes from the =hepatic plexus=, composed of branches
from the vagus and sympathetic nerves.