Abdomen

Jan 27, 2023

The chamber enclosed by the abdominal wall contains a single large peritoneal cavity, which freely communi- cates with the pelvic cavity. Conceptual overview.
34 pages

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pelvic wall at the pelvic inlet. SuperÞcially, the inferior limit ofthe abdominal wall is the superior margin ofthe lower limb. The chamber enclosed by the abdominal wall contains a single large peritoneal cavity , which freely communi- cates with the pelvic cavity. The abdomen is a roughly cylindrical chamber extending from the inferior margin ofthe thorax to the superior mar- gin ofthe pelvis and the lower limb (Fig. 4.1A). The inferior thoracic aperture forms the superior opening to the abdomen, and is closed by the diaphragm. Inferiorly, the deep abdominal wall is continuous with the DiaphragmAInferior thoracic aperture Iliac crest Pelvic inletAbdominal wallInguinal ligamentLower limb 217-360_Ch04_Drake 4/14/04 3:28 PM Page 218

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ˇ Abdominal viscera are either suspended in the peri- toneal cavity by mesenteries or are positioned between the cavity and the musculoskeletal wall (Fig. 4.1B). Abdominal viscera include: major elements ofthe gastrointestinal systemÑthe caudal end ofthe esophagus, stomach, small and large intestines, liver, pancreas, and gallbladder; the spleen; components ofthe urinary systemÑkidneys and ureters; the suprarenal glands; major neurovascular structures. The abdomen houses major elements ofthe gastrointesti- nal system (Fig. 4.2), as well as the spleen and parts ofthe urinary system. Much ofthe liver, gallbladder, stomach, and spleen, and parts ofthe colon are under the domes ofthe diaphragm, which project superiorly above the costal margin ofthe thoracic wall, and as a result these abdominal viscera are protected by the thoracic wall. The superior poles ofthe kidneys are deep to the lower ribs. Viscera not under the domes ofthe diaphragm are sup- ported and protected predominantly by the muscular walls ofthe abdomen. Costal margin MesenteryAortaLeft kidneyRight kidneyMusclesInferior vena cavaPeritoneal cavityB 217-360_Ch04_Drake 4/14/04 3:29 PM Page 219

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ˇ One ofthe most important roles ofthe abdominal wall is to assist in breathing: it relaxes during inspiration to accommodate expan- sion ofthe thoracic cavity and the inferior displace- ment ofabdominal viscera during contraction ofthe diaphragm (Fig. 4.3); during expiration, it contracts to assist in elevating the domes ofthe diaphragm thus reducing thoracic vol- ume. Material can be expelled from the airway by forced expi- ration using the abdominal muscles, as in coughing or sneezing. Contraction ofabdominal wall muscles can dramatically increase intra-abdominal pressure when the diaphragm is in a Þxed position (Fig. 4.4). Air is retained in the lungs by closing valves in the larynx ofthe neck. Increased intra- abdominal pressure assists in voiding the contents ofthe bladder and rectum and in giving birth. InspirationExpiration DiaphragmRelaxation of diaphragmContraction ofdiaphragmContraction of abdominal musclesRelaxation ofabdominalmuscles Laryngeal cavity closedAir retained in thoraxFixed diaphragmContraction of abdominal wallIncrease in intraÐabdominal pressure MicturitionChild birthDefecation 217-360_Ch04_Drake 4/14/04 3:29 PM Page 221

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lateral to the vertebral column, the quadratus lumbo- rum, psoas major, and iliacus muscles reinforce the posterior aspect ofthe wallÑthe distal ends ofthe psoas and iliacus muscles pass into the thigh and are major ßexors ofthe hip joint; lateral parts ofthe abdominal wall are predominantly formed by three layers ofmuscles, which are similar in orientation to the intercostal muscles ofthe thoraxÑ transversus abdominis, internal oblique, and external oblique;anteriorly, a segmented muscle (the rectus abdominis) on each side spans the distance between the inferior thoracic wall and the pelvis. Structural continuity between posterior, lateral, and anterior parts ofthe abdominal wall is provided by thick The abdominal wall consists partly ofbone but mainly of muscle (Fig. 4.5). The skeletal elements ofthe wall (Fig. 4.5A) are: the Þve lumbar vertebrae and their intervening inter- vertebral discs; the superior expanded parts ofthe pelvic bones; bony components ofthe inferior thoracic wall includ- ing the costal margin, rib XII, the end ofrib XI and the xiphoid processMuscles make up the rest ofthe abdominal wall (Fig. 4.5B): External oblique Internal oblique Transversus abdominisPsoas majorRectus abdominisGap between inguinalligament and pelvic boneQuadratuslumborumIliacusCostal margin Rib XIIIliolumbarligamentPelvic inletInguinal ligamentAB 217-360_Ch04_Drake 4/14/04 3:30 PM Page 222

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MesenteryVisceral peritoneum Parietal peritoneumArtery to gastrointestinal tract Retroperitoneal structures Mesentery before fusion with wall Intraperitoneal part of tractSecondary retroperitoneal part of tract ABC ) 217-360_Ch04_Drake 4/14/04 3:31 PM Page 224

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ˇ The superior aperture ofthe abdomen is the inferior tho- racic aperture, which is closed by the diaphragm (see p. 000). The margin ofthe inferior thoracic aperture consists ofvertebra TXII, rib XII, the distal end ofrib XI, the costal margin, and the xiphoid process ofthe sternum. The musculotendinous diaphragm separates the abdomen from the thorax. The diaphragm attaches to the margin ofthe inferior thoracic aperture, but the attachment is complex posteri- orly and extends into the lumbar area ofthe vertebral col- umn (Fig. 4.8). On each side, a muscular extension (crus) Þrmly anchors the diaphragm to the anterolateral surface ofthe vertebral column as far down as vertebra LIII on the right and vertebra LII on the left. Because the costal margin is not complete posteriorly, the diaphragm is anchored to arch-shaped (arcuate) liga- ments, which span the distance between available bony points and the intervening soft tissues; medialand lateral arcuate ligaments cross muscles ofthe posterior abdominal wall and attach to verte- brae, the transverse processes ofvertebra LI and rib XII, respectively; a median arcuate ligament crosses the aorta and is continuous with the crus on each side. The posterior attachment ofthe diaphragm extends much further inferiorly than the anterior attachment. Consequently, the diaphragm is an important component ofthe posterior abdominal wall, to which a number ofvis- cera are related. Esophageal openingCostal margin Median arcuate ligament Medial arcuate ligament Lateral arcuate ligamentLeft crusRight crusQuadratus lumborumPsoas major 217-360_Ch04_Drake 4/14/04 3:33 PM Page 225

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The abdominal wall is continuous with the pelvic wall at the pelvic inlet, and the abdominal cavity is continuous with the pelvic cavity. The circular margin ofthe pelvic inlet is formed entirely by bone: posteriorly by the sacrum; anteriorly by the pubic symphysis; laterally, on each side, by a distinct bony rim on the pelvic bone (Fig. 4.9). Because ofthe way in which the sacrum and attached pelvic bones are angled posteriorly on the vertebral col- umn, the pelvic cavity is not oriented in the same vertical plane as the abdominal cavity. Instead, the pelvic cavity projects posteriorly, and the inlet opens anteriorly and somewhat superiorly (Fig. 4.10). The abdomen is separated from the thorax by the dia- phragm. Structures pass between the two regions through or posterior to the diaphragm (see Fig. 4.8). The pelvic inlet opens directly into the abdomen and struc- tures pass between the abdomen and pelvis through it. The peritoneum lining the abdominal cavity is continu- ous with the peritoneum in the pelvis. Consequently, the abdominal cavity is entirely continuous with the pelvic cavity (Fig. 4.11). Ala of sacrumS IL VPelvic inletInguinal ligamentFalse pelvis Thoracic wallAxis of abdominal cavityAbdominal cavityPelvic cavityPelvic inletAxis of pelvic cavity 217-360_Ch04_Drake 4/14/04 3:34 PM Page 226

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ˇ The abdomen communicates directly with the thigh through an aperture formed anteriorly between the infe- rior margin ofthe abdominal wall (marked by the inguinal ligament) and the pelvic bone (Fig. 4.12). Structures that pass through this aperture are: the major artery and vein ofthe lower limb; the femoral nerve, which innervates the quadriceps femoris muscle, which extends the knee; lymphatics; the distal ends ofpsoas major and iliacus muscles, which ßex the thigh at the hip joint. As vessels pass inferior to the inguinal ligament, their names changeÑthe external iliac artery and vein ofthe abdomen become the femoral artery and vein ofthe thigh. RectumPeritoneumPelvic inletShadow of internal iliac vesselsBladderUterusShadow of ureter 217-360_Ch04_Drake 4/14/04 3:35 PM Page 227

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