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 oesophageal 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 

 oval 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 

 (ca. 5 cm.) long and about half an inch (ca. 1 to 1.5 cm.) mde. 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 cm.) 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. It is relatively large and weighs 2^ to 3 pounds (ca. 1.25 kg.). It is thick and is stronglj' 

 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 hver; it is 

 in the edge of the falciform ligament, which at this time extends to the umbihcus. 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 laminse into the ligaments, and also trabeculse 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 

 hepatic artery both divide into interlobular branches, which run together in the 



1 Flower and Ruge describe the mammalian hver 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 distinctly 

 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. 



