Chapter 59: Abdomen & Pelvis Surface Anatomy

The structural integrity is maintained by a sophisticated musculoskeletal framework, including the five lumbar vertebrae, the respiratory diaphragm superiorly, and the anterior (e.g., rectus abdominis) and posterior (e.g., psoas major) abdominal wall muscles, which collectively enable trunk movement and rotation. Neurovascular communication interfaces are detailed, specifically the thoracoabdominal interface at the respiratory diaphragm (featuring the caval foramen and aortic hiatus) and the pelvis–lower limb interface (involving the greater/lesser sciatic foramina and the obturator foramen). A substantial section outlines the autonomic nervous system's control over visceral function, contrasting sympathetic effects (generally causing vasoconstriction, sphincter contraction, and inhibition of motility/secretion) with opposing parasympathetic effects. Sympathetic input originates from the thoracic splanchnic nerves (greater, lesser, least) and the lumbar and sacral sympathetic trunks, which converge into structures like the ganglion impar. Parasympathetic input comes from the vagus nerves (supplying foregut and midgut derivatives) and the pelvic splanchnic nerves (S2–S4), which innervate hindgut structures and supply vasodilator fibers to erectile tissues. These fibers integrate across major abdominal aortic plexuses, including the coeliac, superior mesenteric, and the superior and inferior hypogastric plexuses. Visceral vascular supply stems from the unpaired branches of the abdominal aorta: the coeliac trunk (foregut), superior mesenteric artery (midgut), and inferior mesenteric artery (hindgut). Venous drainage forms the unique hepatic portal system, collecting blood from the abdominal GI tract and derived organs (excluding the liver itself) before filtering through the hepatic sinusoids. The chapter examines the general microstructure of the gastrointestinal wall, describing the four primary layers: mucosa, submucosa, muscularis externa, and serosa/adventitia. Intrinsic control is managed by the enteric nervous system through the myenteric (Auerbach’s) plexus and the submucosal (Meissner’s) plexus, with specialized Interstitial Cells of Cajal (ICCs) acting as electrical pacemakers for smooth muscle motility and peristalsis. Finally, crucial surface anatomy is established using reference planes—such as the transpyloric plane, often approximating the L1 vertebral body or L1-L2 disc, and the supracristal plane at L4—to accurately localize key viscera, including the liver, spleen, and the abdominal aortic bifurcation, which is essential for guiding physical examinations and clinical procedures like laparoscopy and suprapubic catheterization.