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The liver is divided into eight major subsegments or areas (including the caudate lobe), with the principal line (Cantlie's line) of division between the right and left sides extending cephalad and obliquely from the middle of the gallbladder fossa to the center of the inferior vena cava between the right and left main hepatic veins (Figure 1, A–A′). The true anatomic left lobe thus defined is divided into medial and lateral segments approximately along the line of the falciform or round ligament, and each of these segments is then subdivided into a superior (cephalad) area and an inferior (caudad) area (Figure 2). In contrast, the right lobe is divided into anterior and posterior segments by a plane from the anteroinferior edge of the liver that extends both superiorly and posteriorly. This cleavage is similar to the oblique fissure above the right lower lobe of the lung, and it is roughly parallel to it. These segments of the right hepatic lobe are then split into superior and inferior areas similar to those on the left (Figure 2).

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Although the segmentation of the liver appears straightforward, successful segmentectomy or lobectomy depends upon a thorough understanding of the difference between the portal vein, biliary duct, and hepatic artery distribution as opposed to the hepatic vein drainage. In general, the portal triad structures bifurcate in a serial manner and ultimately lead directly into each of the eight areas. The specific exception to this rule is the paraumbilicalis of the left hepatic branch of the portal vein, as this structure straddles the division between the left inferior medial and lateral segments. Thus, it lies roughly under the round ligament (Figure 1, 7). The superior and inferior areas of the left lateral lobe have a portal venous supply from either end of the paraumbilicalis (Figure 1, 9 and 10); however, special note should be made of the paired medial supply to the superior and inferior areas of the medial segment (Figure 1, 8 and 12). It is equally important at this point to examine the biliary and arterial supply of this area (Figure 6). The main left hepatic duct and artery proceed with the expected bifurcations out through the superior and inferior divisions of the left lateral segment; however, the left medial segment duct ...

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