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The abdomen typically is described topographically using two methods. The first method partitions the abdomen into four quadrants. The second method partitions the abdomen into nine regions.
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The most direct method of partitioning the abdomen is through an imaginary transverse (transumbilical) plane that intersects with a sagittal midline plane through the umbilicus between the L3 and L4 vertebral levels (Figure 7-1A). The two intersecting planes divide the abdomen into four quadrants, described as right and left upper and lower quadrants. The four-quadrant system is straightforward when used to describe anatomic location. For example, the appendix is located in the lower right quadrant of the abdomen.
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For a more precise description, the abdomen is partitioned into nine regions created by two imaginary vertical planes and two imaginary horizontal planes (Figure 7-1B).
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- Vertical planes. Paired vertical planes correspond to the midclavicular lines, which descend to the midinguinal point.
- Subcostal (upper horizontal) plane. Transversely courses inferior to the costal margin, through the level of the L3 vertebra. The L3 vertebra serves as an important anatomic landmark in that it indicates the level of the inferior extent of the third part of the duodenum and the origin of the inferior mesenteric artery.
- Transtubercular (lower horizontal) plane. Transversely courses between the two tubercles of the iliac crest, through the level of the L5 vertebra.
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The transpyloric plane is an imaginary horizontal line through the L1 vertebra, a line that is important when performing radiographic imaging studies. The pylorus of the stomach, the first part of the duodenum, the fundus of the gallbladder, the neck of the pancreas, the origin of the superior mesenteric artery, the hepatic portal vein, and the splenic vein are all located along the level of the transpyloric plane.
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The following structures are helpful anatomic surface landmarks on the anterior abdominal wall (Figure 7-1C):
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- Xiphoid process. The xiphoid process is the inferior projection of the sternum. It marks the dermatome level of T7.
- Umbilicus. The umbilicus lies at the vertebral level between the L3 and L4 vertebrae. However, the skin around the umbilicus is supplied by the thoracic spinal nerve T10 (T10 dermatome) A helpful mnemonic is “T10 for belly but-ten”.
- Inguinal ligament....