438 DIGESTIVE SYSTEM OF THE HORSE 
peritoneum leaving the visceral surface at the portal fissure and along a curved 
line which extends from the fissure to the cesophageal notch. They pass to the 
lesser curvature of the stomach and the first part of the duodenum. 
As stated above, the liver is divided by fissures into three principal lobes— 
right, middle, and left. The right lobe (Lobus dexter) is the largest in the young 
subject and is irregularly quadrilateral in form. On its dorsal part is the caudate 
lobe (Lobus caudatus), which ends in a pointed caudate process directed outward 
and assists in forming the cavity for the right kidney. The central or middle lobe 
(Lobus centralis) is normally much the smallest. The left lobe (Lobus sinister) is 
val in outline and thickest centrally. In old or middle-aged subjects it commonly 
exceeds the right one in size, and in many cases constitutes the bulk of the gland. 
In some cases the atrophy of the right lobe is so extreme that the middle lobe may 
exceed it in size.! 
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 about two inches 
(ea. 5 em.) long and about half an inch (ca. 1 to 1.5 em.) wide. It passes be- 
tween the two layers of the mesoduodenum, and pierces the wall of the duodenum 
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 em.) before opening into the diverticulum duodeni. The arrange- 
ment forms a valve, which prevents regurgitation from the intestine. There is no 
gall-bladder. 
In the new-born foal the liver presents striking differences when compared with the gland of 
the adult. Itis relatively large and weighs 2/4 to 3 pounds (ca. 1.25kg.). It is thick and is strongly 
curved, and a considerable part of the parietal surface is in contact with the floor of the abdomen. 
The umbilical fissure is large and contains the umbilical vein. The latter is a very large vessel 
which carries blood from the placenta and joins the portal vein in the substance of the liver; it is 
in the edge of the falciform ligament, which at this time extends to the umbilicus. The visceral 
surface is deeply concave and is in contact chiefly with the stomach and duodenum. 
Structure.—The liver is covered by an external serous, and an internal fibrous, 
coat. The serous coat (Tunica serosa) covers the gland except at the attachment 
of the pancreas and at the portal fissure; it is reflected from it to form the ligaments 
and the lesser omentum. The fibrous capsule (Capsula fibrosa) is in general thin; 
it sends lamine into the ligaments, and also trabeculz into the gland. 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 polygonal lobules (Lobuli hepatis), about 
1.5 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; for the same reason the 
organ is also quite friable. The lobules are composed of the polyhedral liver cells, 
a delicate reticulum, the bile capillaries, a plexus of blood capillaries, and a central 
vein. 
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 
Sete 
. . . . . . . | 
hepatic artery both divide into interlobular branches, which run together in the 
1 Flower and Ruge describe the mammalian liver as being primarily divided by the umbilical 
fissure into two parts, the right and left lobes. Secondary fissures on either side may subdivide 
each of these primary lobes. On this basis we may recognize in the liver of the horse right lateral, 
right central, left central, and left lateral lobes. In the young foal these four lobes are distinetly 
recognizable. The two central lobes would correspond to the central or middle lobe of the foregoing | 
description, and the right central lobe would be the equivalent of the quadrate lobe of man. 
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