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LIVER (O. Eng. lifer; cf. cognate for...

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Originally appearing in Volume V16, Page 802 of the 1911 Encyclopedia Britannica.
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LIVER (O. Eng. lifer; cf. cognate forms, Dutch lever, Ger. Leber, Swed. lefver, &c.; the O. H. Ger. forms are libara, lipora, &c.; the Teut. word has been connected with Gr. i'prap and Lat. jecur), in anatomy, a large reddish-brown digestive gland situated in the upper and right part of the abdominal cavity. When hardened in situ its shape is that of a right-angled, triangular prism showing five surfaces—superior, anterior, inferior, posterior and right lateral which represents the base of the prism. It weighs about three pounds or one-fortieth of the body weight. Although the liver is a fairly solid organ, it is plastic, and moulds itself to even hollow neighbouring viscera rather than they to it. The superior surface is in contact with the diaphragm, but has peritoneum between (see COELOM AND SEROUS MEMBRANES). At its posterior margin the peritoneum of the great sac is reflected on to the diaphragm to form the anterior layer of the coronary ligament. Near the mid line of the body, and at right angles to the last, another reflection, the falciform ligament, runs forward, and the line of attachment of this indicates the junction of the right and left lobes of the liver. The anterior surface is in contact with the diaphragm and the anterior abdominal wall. The attachment of the falciform ligament is continued down it. The posterior surface is more complicated (see fig. 1); starting from the right and working toward the left, a large triangular area, uncovered by peritoneum and in direct contact with the diaphragm, is seen. This is bounded on the left by the inferior vena cava, which is sunk into a deep groove in the liver, and into the upper part of this the hepatic veins open. Just to the right of this and at the lower part of the bare area is a triangular depression for the right supra-renal body. To the left of the vena cava is the Spigelian lobe, which lies in front of the bodies of the tenth and eleventh thoracic vertebrae, the lesser sac of peritoneum, diaphragm and thoracic aorta intervening. To the left of this is the fissure for the ductus venosus, and to the left of this again, the left lobe, in which a broad shallow groove for the oesophagus may usually be seen. Sometimes the left lobe stretches as far as the left abdominal wall, but more often it ends below the apex of the heart, which is 3z in. to the left of the mid line of the body. The relations of the lower surface can only be understood if it is realized that it looks backward and to the left as well as downward (see fig. 1). Again starting from the right side, two impressions are seen; the anterior one is for the hepatic flexure of the colon, and the posterior for the upper part of the right kidney. To the left of the colic impression is a smaller one for the second part of the duodenum. Next comes the gall bladder, a pear-shaped bag, the fundus of which is in front and below, the neck behind and above. From the neck passes the cystic duct, which is often twisted into the form of an S. To the left of the gall bladder is the quadrate lobe, which is in contact with the pylorus of the stomach. To the left of this is the left lobe of the liver, separated from the quadrate lobe by the umbilical fissure in which lies the round ligament of the liver, the remains of the umbilical vein of the foetus. Sometimes this fissure is partly turned into a tunnel by a bridge of liver substance known as the pons hepatis. Yvt. 26The under surface of the left lobe is concave for the interior surface of the stomach (see ALIMENTARY CANAL: Stomach Chamber), while a convexity, known as the tuber omentale, fits into the lesser curvature of that organ. The posterior boundary of the quadrate lobe is the transverse fissure, which is little more than an inch long and more than half an inch wide. This fissure represents the hilum of the liver, and contains the right and left hepatic ducts and the right and left branches of the hepatic artery and portal vein, together with nerves and lymphatics, the whole being enclosed in some condensed subperitoneal tissue known as Glisson's capsule. Behind the transverse fissure the lower end of the Spigelian lobe is seen as a knob called the tuber papillare, and from the right of this a narrow bridge runs forward and to the right to join the Spigelian lobe to the right Vena cava in its fossa lobe and to shut off the transverse fissure from that for the vena cava. This is the caudate lobe. The right surface of the liver is covered with peritoneum and is in contact with the diaphragm, outside which are the pleura and lower ribs. From its lower margin the right lateral ligament is reflected on to the diaphragm. A similar fold passes from the tip of the left lobe as the left lateral ligament, and both these are the lateral margins of the coronary ligament. Sometimes, especially in women, a tongue-shaped projection downward of the right lobe is found, known as Riedel's lobe; it is of clinical interest as it may be mistaken for a tumour or floating kidney (see C. H. Leaf, Proc: Anat. Soc., February 1899; Journ. Anat. and Phys. vol. 33, p. ix.). The right and left hepatic ducts, while still in the transverse fissure, unite into a single duct which joins the cystic duct from the gall bladder at an acute angle. When these have united the II Gall bladder Duodenal impression Colic impression From A. Birmingham Cunningham's Text-book of Anatomy. End of right suprarenal vein Suprarenal impression I Right end of caudate lobe Uncovered area of right lobe Renal impression Attachment of right lateral ligament duct is known as the common bile duct, and runs down to the second part of the duodenum (see ALIMENTARY CANAL). Minute Structure of the Liver.—The liver is made up of an enormous number of lobules of a conical form (see fig. 3). If the portal vein is followed from the transverse fissure; it will be seen to branch and re-branch until minute twigs called interlobular veins (fig. 2, i) ramify around the lobules. From these intralobular capillaries run toward the centre of the lobule, forming a network among the polygonal hepatic cells. On reaching the core of the conical lobule they are collected into a central or intralobular vein (fig. 2, c) which unites with other similar ones to form a sublobular vein (fig. 3, s). These eventually reach the hepatic radicles, and so the blood is conducted into the vena cava. In man the lobules are not distinctly separated one from the other, but in some animals, e.g. the pig, each one has a fibrous sheath derived from Glisson's capsule(fig. 3, CO. Embryology.—The liver first appears as an entodermal hollow longitudinal outgrowth from the duodenum into the ventral mesentery. The upper part of this forms the future liver, and grows up into the septum transversum from which the central part of the diaphragm is formed (see DIA PH RAGM). From the cephalic part of this primary diverticulum solid rods of cells called the hepatic cylinders grow out, and these branch again and again until a cellular network is formed surrounding and breaking up the umbilical and vitelline veins. The liver cells, therefore, are entodermal, but the supporting connective tissue mesodermal from the septum transversum. The lower (caudal) part of the furrow-like outgrowth remains hollow and forms the gall bladder. At first the liver is em-bedded in the septum transversum, but later the diaphragm and it are constricted off one from the other, and soon the liver becomes very large and fills the greater part of the abdomen. At birth it is proportionately much larger than in the adult, and forms one-eighteenth instead of one-fortieth of the body weight, the right and left lobes being nearly equal in size. Comparative Anatomy.—In the Acrania (Amphioxus) the liver is probably represented by a single ventral diver- ticulum from the anterior end of the intestine, which has a hepatic portal circula- tion and secretes digestive fluid. In all the Craniata a solid liver is developed. In the adult lamprey among the Cyclostomata the liver under- 3.-Vertical section through two goes retrogression, and the hepatic lobules of a pig. bile ducts and gall bladder disappear, though they are present in the larval form (Ammocoetes). In fishes and amphibians the organ consists of right and left lobes, and a gall-bladder is present. The same description applies to the reptiles, but a curious net-work of cystic ducts is found in snakes and to a less extent in crocodiles. In the Varanidae (Monitors) the hepatic duct is also retiform (see F. E. Beddard, Proc. Zool. Soc., 1888, p. 105). In birds two lobes are also present, but in some of them, e.g. the pigeon, there is no gall-bladder. In mammals Sir William Flower pointed out that a generalized type of liver exists, from which that of any mammal may be derived by suppression or fusion of lobes. The accompanying diagram of Flower (fig. 4) represents an ideal mammalian liver. It will be seen that the umbilical fissure (u) divides the organ into right and left halves, as in the lower vertebrates, but that the ventral part of each half is divided into a central and lateral lobe. Passing from right to left there are therefore: right lateral (rl), right central (rc), left central (lc), and left lateral (II) lobes. The gall-bladder (g), when it is present, is always situated on the caudal surface or in the substance of the right central lobe. The Spigelian (s) and caudate lobes (c) belong to the right half of the liver, the latter being usually a leaf-shaped lobe attached by its stalk to the Spigelian, and having its blade flattened between the right lateral lobe and the right kidney. The vena cava (vc) is always found to the right of the Spigelian lobe and dorsal to the stalk of the caudate. In tracing the lobulation of man's liver back to this generalized type, it is evident at once that his quadrate lobe does not correspond to any one generalized lobe, but is merely that part of the right central which lies between the gall bladder and the umbilical fissure. From a careful study of human variations (see A. Thomson, Journ. Anat. and Phys. vol. 33, p. 546) compared with an Anthropoid liver, such as that of the gorilla, depicted by W. H. L. Duckworth (Morphology and Anthro- u, Umbilical vein of the foetus, rlf,The right lateral fissure. represented by the round cf, The cystic fissure. ligament in the adult, lying 11, The left lateral lobe. in the umbilical fissure. lc, The left central lobe. dv, The ductus venosus. rc, The right central lobe. vc, The inferior vena cava. rl, The right lateral lobe. p, The vena portae entering the s, The Spigelian lobe. transverse fissure. c, The caudate lobe. llf, The left lateral fissure. g, The gall bladder. pology, Cambridge, 1904, p. 98), it is fairly clear that the human liver is formed, not by a suppression of any of the lobes of the generalized type, but by a fusion of those lobes and obliteration of certain fissures. This fusion is, probably correctly, attributed by Keith to the effect of pressure following the assumption of the erect position (Keith, Proc. Anat. Soc. of Gt. Britain, Journ. Anat. and Phys. vol. 33, p. xii.). The accom- panying diagram (fig. 5) shows an abnormal human liver in the Anatomical Department of St Thomas's Hospital which reproduces the generalized type. In its lobulation it is singularly like, in many details, that of the baboon (Papio maimon) figured by G. Ruge (Morph. Jahrb., Bd. 35, p. 197); see F. G. Parsons, Proc. Anat. Soc., Feb. 1904, Journ. Anat. and Phys. vol. 33, p. xxiii. Georg Ruge " Die ausseren Formverhaltnisse der Leber bei den Primaten," (Morph. Jahrb., Bd. 29 and 35) gives a critical study of the primate liver, and among other things suggests the re-cognition of the Spigelian and caudate lobes as parts of a single lobe, for which he proposes the name of lobus venae cavac. This doubtless would be an advantage morphologically, though for human descriptive anatomy the present nomenclature is not likely to be altered. The gall-bladder is usually present in mammals, but is wanting in the odd-toed ungulates (Perissodactyla) and Procavia (Hyrax). In the giraffe it may be absent or present. The cetacea and a few rodents are also without it. In the otter the same curious network of bile ducts already recorded in the reptiles is seen (see P. H. Burne, Proc. Anat. Soc., Journ. Anat. and Phys. vol. 33, p. xi.). (F. G. P.)
End of Article: LIVER (O. Eng. lifer; cf. cognate forms, Dutch lever, Ger. Leber, Swed. lefver, &c.; the O. H. Ger. forms are libara, lipora, &c.; the Teut. word has been connected with Gr. i'prap and Lat. jecur)

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