Abdomen and pelvis: overview and surface anatomy

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CHAPTER 60 Abdomen and pelvis: overview and surface anatomy

GENERAL STRUCTURE AND FUNCTION OF THE ABDOMINOPELVIC CAVITY

Although often considered separately, the abdomen and pelvis form the largest effectively continuous visceral cavity of the body. They act together to provide multiple vital functions including: support and protection of the digestive and urinary tracts and internal reproductive organs and their associated neurovascular supplies; transmission of the neurovascular supply to and from the thorax and the lower limb; provision of support and attachment to the external genitalia and access to and from the internal reproductive and urinary organs; provision of accessory muscles of physiological actions such as respiration, defecation, and micturition; support for the spinal column in weight bearing and movement.

MUSCULOSKELETAL FRAMEWORK OF THE ABDOMEN AND PELVIS

The walls of the abdominopelvic cavity consist of five lumbar vertebrae and their intervening intervertebral discs (lying in the posterior midline); three layers of skeletal muscles (transversus abdominis, internal oblique and external oblique) with associated fasciae and skin (lying lateral and anterolateral); a single muscular layer (rectus abdominis) with its associated fascial coverings (lying anterior); the bony ‘bowl’ formed by the walls of the true and false pelvis (ilium, ischium and pubis on each side); the muscles of the pelvic floor and perineum (lying inferiorly); the diaphragm (lying superiorly) (Fig. 60.1).

The bony protection of the cavity is restricted to the pelvis (true and false) and the upper abdomen, which is partly enclosed by the anterolateral portions of the lower six ribs and their cartilages even though these structures are technically part of the thoracic wall. Between these two zones, the anterolateral abdominal wall is entirely musculofascial but of sufficient thickness and strength that it provides adequate protection for the viscera such that even direct blows can be resisted.

The abdominal wall and retroperitoneal structures play an important role in the function of the spinal column in both movement of the thorax in relation to the pelvis and in aiding support of the spine in weight bearing. The anterolateral muscles provide assistance with flexion and rotation of the thorax in relation to the pelvis (or vice versa if the thorax is fixed).

In the upright position, the pelvis lies at an angle such that the pelvic canal slopes posteriorly at an angle of 45° to the main abdominal visceral cavity. The main abdominal cavity is shaped by the spinal column and the posterior abdominal wall muscles. Although less pronounced than in the thorax, due to the presence of quadratus lumborum and psoas major, its prominence within the abdominal cavity produces two distinct paravertebral gutters on either side of the vertebral column. The lordosis of the lumbar spine in combination with the angle between the fifth lumbar vertebra and the sacrum produces the parabola shape of the sagittal section of each paravertebral gutter.

Thoracoabdominal interface

The thoracic and abdominal cavities interface with each other across the diaphragm. Six principle communications exist between the two cavities: the inferior vena cava, as it crosses between its short supra-hepatic portion and the intrathoracic portion below the right atrium, through the caval opening of the diaphragm (Ch. 62); the oesophagus passing inferiorly through the oesophageal opening of the diaphragm (Ch. 65); the aorta between the descending thoracic and abdominal portions, posterior to the median arcuate ligament of the diaphragm (Ch. 62); the lymphatics of the abdomen draining upwards to the thorax, mainly posterior to the median arcuate ligament via the thoracic duct lying posterolateral to the aorta, but also via peri-caval lymphatics and small vessels draining directly through and at the peripheral insertions of the diaphragm; the azygos and hemiazygos veins ascending behind the median arcuate ligament of the diaphragm into the thoracic azygos system (Ch. 62); the autonomic nervous system, both sympathetic and parasympathetic (see below) which cross between the two cavities behind the medial arcuate ligaments of the diaphragm, via the various diaphragmatic openings, and directly through the substance of the diaphragm itself (Ch. 54). Cutaneous neurovascular structures also cross between the thorax into the abdominal wall structures.

Pelvis–lower limb interface

The pelvis forms an integral part of the bony structure of both the abdominopelvic cavity and the lower limb. It transmits the weight of the upright body from the femoral heads to the lumbar spine as well as providing a stable platform about which movement of the hip joint can occur. Its bony surfaces give rise to extensive muscular attachments for the muscles of the buttock and thigh (Ch. 67) as well as for the muscles of the pelvic floor and perineal diaphragm (Ch. 63). The pelvis also transmits or gives origin to the neuro-lympho-vascular structures which supply the lower limb. The principle structures are as follows. The venous drainage includes the common femoral vein passing under the inguinal ligament to become the external iliac vein; the obturator veins passing through the obturator foramen; the superior and inferior gluteal veins draining from the buttock into the internal iliac vein via the greater and lesser sciatic foramina; multiple smaller venous channels. The arterial supply includes the external iliac artery passing under the inguinal ligament to become the common femoral artery; the obturator artery passing into the thigh via the obturator foramen; the superior and inferior gluteal arteries from the posterior division of the internal iliac artery passing out of the pelvis via the greater and lesser sciatic foramina. The lymphatic drainage principally passes under the inguinal ligament, via the femoral and obturator canals and via the sciatic foramina, to drain into the pelvic lymphatic chains (Ch. 63). The somatic neural structures include the femoral, obturator and sciatic nerves and other minor branches of the lumbosacral plexus (Ch. 62). The autonomic neural structures include those running with the arterial supply and with the branches of the lumbosacral plexus. Cutaneous neurovascular structures also cross between the lower abdomen and pelvis into the skin and superficial tissues of the lower limb.

GENERAL ARRANGEMENT OF ABDOMINOPELVIC NEUROVASCULAR STRUCTURES

It is useful to review here the overall arrangement of the neurovascular systems which cover several regions or multiple viscera of the abdominopelvic region. Descriptions of the neurovascular supply to individual organs are given in the relevant chapters.

AUTONOMIC SUPPLY

The autonomic supply to the abdominal and pelvic viscera is via the abdominopelvic part of the sympathetic chain and the greater, lesser and least splanchnic nerves (sympathetic), and the vagus and pelvic parasympathetic nerves (parasympathetic). Considerable ramification occurs between plexuses and ganglia, particularly in the major plexuses around the abdominal aorta, hence the descriptions tend to be simplifications based on the ‘main’ supply to each organ (Fig. 60.2). The details of the terminations of these fibres are given on page 1050.

Sympathetic innervation

The cell bodies of neurones of the sympathetic supply of the abdomen and pelvis lie in the intermediolateral grey matter of the first to 12th thoracic and first two lumbar spinal segments. These neurones give rise to myelinated axons which travel in the ventral ramus of the spinal nerve of the same level, leaving it via the white ramus communicans to enter a thoracic or lumbar paravertebral ganglion. Visceral branches may exit at the same level or ascend or descend several levels before exiting but leave the ganglia without synapsing in medial (visceral) branches. These give rise to the greater, lesser and least splanchnic nerves, and the lumbar and sacral splanchnic nerves. Axons destined for supply to somatic structures always synapse in the ganglion of the same level and post-ganglionic, unmyelinated axons leave the ganglion via the grey ramus communicans to enter the spinal nerve of the same level.

Lumbar sympathetic system

The lumbar part of each sympathetic trunk usually contains four interconnected ganglia. The trunk runs in the extraperitoneal connective tissue anterior to the vertebral column and along the medial margin of psoas major. Superiorly, it is continuous with the thoracic trunk posterior to the medial arcuate ligament. Inferiorly, it passes posterior to the common iliac artery and is continuous with the pelvic sympathetic trunk. On the right side, it lies posterior to inferior vena cava, and on the left it is posterior to the lateral aortic lymph nodes. It is anterior to most of the lumbar vessels, but may pass behind some lumbar veins.

The first, second and sometimes third lumbar ventral spinal rami send white rami communicantes to the corresponding ganglia. Grey rami communicantes pass from all four lumbar ganglia to the lumbar spinal nerves. They are long, and accompany the lumbar arteries round the sides of the vertebral bodies, medial to the fibrous arches to which psoas major is attached. They supply the sympathetic innervation to the lumbar somatomes via these arteries. Four lumbar splanchnic nerves pass as medial branches from the ganglia to join the coeliac, inferior mesenteric (or occasionally abdominal aortic) and superior hypogastric plexuses. The first lumbar splanchnic nerve, from the first ganglion, gives branches to the coeliac, renal and inferior mesenteric plexuses. The second nerve joins the inferior part of the intermesenteric or inferior mesenteric plexus. The third nerve arises from the third or fourth ganglion and passes anterior to the common iliac vessels to join the superior hypogastric plexus. The fourth lumbar splanchnic nerve from the lowest ganglion passes above the common iliac vessels to join the lower part of the superior hypogastric plexus, or the inferior hypogastric ‘nerve’.

Pelvic sympathetic system

The pelvic sympathetic trunk lies in the extraperitoneal tissue anterior to the sacrum beneath the presacral fascia. It lies medial or anterior to the anterior sacral foramina and has four or five interconnected sacral ganglia (they often merge to a greater or lesser extent). Above, it is continuous with the lumbar sympathetic trunk and receives preganglionic fibres descending via the lumbar chain from the lower lumbar spinal segments. Below the lowest ganglia, the two trunks converge to unite in the small ganglion impar anterior to the coccyx. Grey rami communicantes pass from the ganglia to the sacral and coccygeal spinal nerves but there are no white rami communicantes. Medial branches connect across the midline and twigs from the first two ganglia, referred to as sacral splanchnic nerves, join the inferior hypogastric plexus or the hypogastric ‘nerve’ to be distributed to the pelvic viscera via a fine network of pelvic nerves (the ‘pelvic plexus’). Small branches form a plexus on the median sacral artery.

Parasympathetic innervation

The parasympathetic neurones innervating the abdomen and pelvis lie either in the dorsal motor nucleus of the vagus nerve or in the intermediolateral grey matter of the second, third and fourth sacral spinal segments. The vagus nerves supply parasympathetic innervation to the abdominal viscera with the exception of the colon distal to the mid descending colon, rectum and upper anal canal. The nerves are derived from the oesophageal plexus and enter the abdomen via the oesophageal opening, closely related to the anterior and posterior walls of the abdominal oesophagus, from which they are separated by loose connective tissue. The anterior vagus is mostly derived from the left vagus and the posterior from the right vagus. The nerves supply the intra-abdominal oesophagus and stomach directly. The anterior nerve supplies branches to the hepatic plexus, which innervates the liver parenchyma and vasculature, the biliary tree including the gallbladder, and the structures in the free edge of the lesser omentum. The posterior nerve supplies branches to the coeliac plexus. These fibres frequently constitute the largest portion of the fibres derived from the plexus: they arise directly from the nerve and from the greater posterior gastric nerve and run beneath the peritoneum, deep to the posterior wall of the lesser sac, at the upper limit of the lesser omentum to reach the coeliac plexus. Their synaptic relays with postganglionic neurones are situated in the myenteric (Auerbach’s) and submucosal (Meissner’s) plexuses (see below).

Abdominopelvic autonomic plexuses and ganglia

The abdominopelvic autonomic plexuses are somewhat variable and often fuse or are closely inter-related. The following descriptions recognize their main features (Figs 60.3, 60.4).

Superior hypogastric plexus

The superior hypogastric plexus lies anterior to the aortic bifurcation, the left common iliac vein, medial sacral vessels, fifth lumbar vertebral body and sacral promontory, and between the common iliac arteries. It is occasionally termed the presacral nerve, but is seldom a single nerve and it is prelumbar rather than presacral. Most frequently found to the left side of the midline, it lies in extraperitoneal connective tissue from which the parietal peritoneum can easily be stripped. The breadth and condensation of its constituent nerves vary. The attachment of the sigmoid mesocolon and upper limit of the mesorectum containing the superior rectal vessels are anterior and to the left of the lower part of the plexus, and separated from it only by a thin sheet of loose connective tissue. The plexus is formed by branches from the aortic plexus and the third and fourth lumbar splanchnic nerves (which are mainly sympathetic). It may also contain parasympathetic fibres from the pelvic splanchnic nerves, which ascend from the two inferior hypogastric plexuses, via a series of filaments which are sometimes identified as the right and left hypogastric ‘nerves’. The latter lie in loose connective tissue just posterolateral to the start of the mesorectum and pass over the pelvic brim medial to the internal iliac vessels. The superior hypogastric plexus supplies fibres to the inferior mesenteric plexus and to the ureteric, gonadal and common iliac plexuses as well as small branches which turn abruptly forward into the upper mesorectum to supply the plexus around the superior rectal artery.

Inferior hypogastric plexus, pelvic plexus and nerves

The inferior hypogastric plexus lies in the thin extraperitoneal connective tissue on the pelvic side-wall anterolateral to the mesorectal fascia. It is directly related to the internal iliac vessels and the attachments of levator ani, coccygeus and obturator internus which lie laterally and the superior vesical and obliterated umbilical arteries superiorly. In males, the inferior hypogastric plexus lies posterolaterally on either side of the seminal vesicles, prostate and the posterior part of the urinary bladder. In females, each plexus lies lateral to the uterine cervix, vaginal fornix and the posterior part of the urinary bladder, and often extends into the broad ligaments of the uterus.

The inferior hypogastric plexus is formed mainly from the sacral splanchnic (sympathetic) and pelvic splanchnic (parasympathetic) branches; a small contribution is made from some sympathetic fibres (from the lower lumbar ganglia) which descend into the plexus via the inferior hypogastric nerves from the superior hypogastric plexus. It gives origin to a complex network of tiny pelvic branches, often referred to as the ‘pelvic plexus’, which supply the pelvic viscera either directly or via periarterial plexuses. The branches of the pelvic plexus supply the vas deferens, seminal vesicles, prostate, accessory glands and penis in males, the ovary, fallopian tubes, uterus, uterine cervix and vagina in females, and the urinary bladder and distal ureter in both sexes. The plexus also gives rise to the long branches of the parasympathetic pelvic splanchnic nerves.

MAJOR ABDOMINOPELVIC ARTERIAL AND VENOUS ARRANGEMENTS

The major vessels which occupy the abdomen and pelvis not only supply the viscera, retroperitoneal structures and much of the bony and soft tissue walls of the cavities, they also course through the cavities en route to supply the lower limbs. Both the arterial and systemic venous trees of the abdomen and pelvis lie predominantly posteriorly in the abdomen and postero-laterally in the pelvis; from the caval and aortic orifices in the diaphragm and the other points where smaller vessels cross the abdominothoracic interface, they follow the general parabolic shape of the lumbar spine. The pelvic divisions follow the contours of the brim and side wall of the true pelvis (Figs 60.6, 60.7). The individual parts of the aorto-iliac and iliocaval systems are described in the appropriate chapters.

Arterial supply to the gastrointestinal tract

The arterial supply to the gastrointestinal tract is derived from the anterior midline visceral branches of the aorta. There are usually three anterior branches, the coeliac trunk and the superior and inferior mesenteric arteries. Variants in the origin of the arteries are very rare. The most common is a joint origin of the upper two branches as either a coeliacomesenteric trunk or a lienohepaticomesenteric trunk with a separate left gastric artery. Accessory or replaced branches to the upper abdominal viscera are more common (see Chs 65, 66, 67 and 68). Accessory anastomotic vessels between the origin of the coeliac trunk and superior mesenteric artery are rare. The inferior mesenteric artery always arises separately, however, replaced, accessory or anastomotic vessels occasionally arise from the proximal superior mesenteric artery or its branches and contribute to the arterial supply to the proximal territory of the inferior mesenteric artery.

The coeliac trunk and its branches supplies the gastrointestinal tract from the distal third of the oesophagus to the mid part of the duodenum and all derived adenexae (liver, biliary tree, spleen, dorsal pancreas, greater omentum and lesser omentum). The superior mesenteric artery supplies the tract from the mid second part of the duodenum to the distal third of the transverse colon (jejunum, ileum, caecum, appendix, ascending colon and ileal mesentery). The inferior mesenteric artery supplies the tract from the distal transverse colon to the upper anal canal. Other than accessory arteries, numerous medium calibre arteries form anastomoses between the vascular territories. The most profuse occurs around the head of the pancreas and the duodenum, where anastomoses form between the anterior and posterior superior pancreaticoduodenal arteries and the inferior pancreaticoduodenal arteries; the posterior superior pancreaticoduodenal artery and jejunal arteries; the greater pancreatic artery and jejunal arteries. The anastomoses between the territories of the superior and inferior mesenteric arteries are less pronounced and more variable: the most reliable is the pericolic marginal artery, which runs along the transverse colon. Additional anastomotic arcades may exist in the colic mesentery between the transverse and descending colon.

Portal venous system

The (hepatic) portal system, like all portal venous systems, connects two capillary beds: that of the abdominal part of the digestive tube, with the exception of the lower anal canal but including the abdominal part of the oesophagus, and all organs, except the liver, derived from it (i.e. the spleen, pancreas and gallbladder) and the hepatic sinusoidal ‘capillary’ bed. The intrahepatic portal vein ramifies like an artery, and ends in sinusoids from which vessels again converge to reach the inferior vena cava via the hepatic veins. In adults, the portal vein and its tributaries have no valves. In fetal life and for a short postnatal period, valves are demonstrable in its tributaries, but they usually atrophy; although some occasionally persist in an atrophic form into adulthood.

Portal vein

The portal vein begins at the level of the second lumbar vertebra and is formed from the convergence of the superior mesenteric and splenic veins (see Fig. 70.8). It is approximately 8 cm long and lies anterior to the inferior vena cava, posterior to the neck of the pancreas (Fig. 60.8) and obliquely to the right. It ascends behind the first part of the duodenum, the common bile duct and gastroduodenal artery. At this point it is directly anterior to the inferior vena cava. It enters the right border of the lesser omentum, ascends anterior to the epiploic foramen to reach the right end of the porta hepatis, and then divides into right and left main branches which accompany the corresponding branches of the hepatic artery into the liver. In the lesser omentum it lies posterior to both the common bile duct and hepatic artery. The portal vein is surrounded by the hepatic nerve plexus and accompanied by many lymph vessels and some lymph nodes.

GENERAL MICROSTRUCTURE OF THE GUT WALL

The gut wall displays a common structural plan which is modified regionally to take account of local functional differences. The general microstructure is best appreciated by reference to the development of the gut (Ch. 73). Much of the alimentary canal originates as a tube of endoderm enclosed in splanchnopleuric mesoderm. Its external surface faces the embryonic coelom, and the endodermal lining forms the epithelium of the canal and also the secretory and ductal cells of various glands which secrete into the lumen, including the pancreas and liver. The splanchnopleuric mesoderm forms the connective tissue, muscle layers, blood vessels and lymphatics of the wall, and its external surface becomes the visceral mesothelium or serosa. There is no serosa surrounding the cervical and thoracic portions of the gut, or where the hindgut traverses the pelvic floor: in these sites the gut tube is surrounded by a connective tissue adventitia. Neural elements invade the gut from neural crest tissue (Ch. 24). The smooth muscle of the muscularis externa layers of the alimentary canal is supplemented with striated muscle both cranially (from the branchial arches) and caudally. The mature gut wall has four main layers, namely mucosa, submucosa, muscularis externa and serosa (Fig. 60.9). The mucosa (mucous membrane) is the innermost layer and is subdivided into a lining epithelium, an underlying lamina propria (a layer of loose connective tissue, where many of the glands are also found) and a thin layer of smooth muscle, the muscularis mucosae. The submucosa is a strong and highly vascularized layer of connective tissue. The muscularis externa consists of inner circular and outer longitudinal layers which are present throughout the gut wall: a partial oblique layer is present only in the stomach. The external surface is bounded by a serosa or adventitia, depending on its position within the body.

Mucosa

Muscularis externa

The muscularis externa usually consists of distinct inner circular and outer longitudinal layers whose antagonistic activities create waves of peristalsis responsible for the movement of ingested material through the lumen of the gut. In the stomach, where movements are more complex, there is a partial oblique layer, internal to the other two layers. The layer of circular muscle is invariably thicker than the longitudinal muscle, except in the colon, where the longitudinal muscle is gathered into three cords (taenia coli).

The muscularis externa is composed almost exclusively of smooth muscle, except in the upper oesophagus, where smooth muscle blends with striated muscle. Although the oesophageal musculature resembles that of the pharynx, it is entirely under involuntary control. For most of its length, the smooth muscle of the gut wall consists of ill-defined bundles of cells, typically visceral in type, and somewhat larger than vascular smooth muscle cells. They are approximately 500 μm long, regardless of body size, and are electrically and mechanically coupled. Their fasciculi lack a perimysium, but have sharp boundaries.

The arrangement of the musculature means that a segment of gut can change extensively in diameter (to virtual occlusion of the lumen) and also in length, although elongation is limited by the presence of mesenteries. The co-ordinated activity of the two muscle layers produces a characteristic motor behaviour which is mainly propulsive and directed anally (peristalsis), combined with a non-propulsive motor activity which either mixes the luminal contents, as occurs in the stomach, or partitions them, as occurs at the pyloric sphincter. The muscle maintains constant volume, so that shortening of a segment of the gut wall is accompanied by an increase in muscle layer circumference.

Intestinal smooth muscle exhibits variable and changing degrees of contraction on which rhythmic (or phasic) contractions are superimposed. Slow waves of rhythmic electrical activity, driven by changes in membrane potential in pacemaker cells (interstitial cells), spread throughout the thickness of the circular and longitudinal smooth muscle coats. After spreading circumferentially, slow waves can move in either oral or anal directions, causing segmental contraction. The distances of propagation and the patterns of this spontaneous activity vary between areas of the intestine. Neural regulation of slow and phasic contractions involves excitatory and inhibitory transmitters which are released from the myenteric plexus. This motor control is closely co-ordinated with mucosal absorption and secretion and is mediated via intrinsic nerves in the submucous plexus. The peristaltic reflex occurs during passage of luminal contents down the intestine. It involves ascending contraction and descending relaxation: the sensory limb is mediated by sensory neurones that respond to either mucosal stimulation (intrinsic primary afferents) or muscle stretch (extrinsic afferents).

Innervation

The gut is densely innervated by the autonomic and enteric nervous systems, and is under extrinsic and intrinsic neuronal control. Neuronal cell bodies of the enteric nervous system lie between the circular and longitudinal components of the muscularis externa (myenteric plexus) and within the submucosa (submucosal plexus). They provide the intrinsic sensory and motor supply of the gut wall and connect with extrinsic sensory, motor and sensorimotor nerves of cranial or spinal origin.

Extrinsic innervation

The extrinsic innervation is derived from neurones outside the gut, and contains functional components from the sympathetic, parasympathetic and visceral sensory divisions of the peripheral nervous system (see p. 1041). Visceral sensory endings respond to excessive muscular contraction or distension: their cell bodies are situated in the nodose ganglion of the vagus nerve and in thoracic and lumbar spinal or dorsal root ganglia. The cell bodies of parasympathetic efferent axons lie in the vagal dorsal motor nucleus in the medulla oblongata. Sympathetic efferent fibres arise from the thoracic and lumbar spinal cord and relay in prevertebral sympathetic ganglia (coeliac, mesenteric and pelvic).

A subserosal plexus, which sometimes contains neuronal cell bodies, connects the extrinsic nerve fibres with the myenteric plexus and is particularly prominent near the mesentery. Fibres from this plexus run through the longitudinal muscle layer to reach the myenteric plexus.

SURFACE AND DIAGNOSTIC ANATOMY OF THE ABDOMEN AND PELVIS

ABDOMINAL PLANES AND REGIONS

For descriptive purposes, the abdomen can be divided by a number of imaginary horizontal and vertical lines drawn using the skeletal landmarks of the thorax and abdomen (Fig.). Projection of these lines into the sagittal or transverse planes can then be used to define certain abdominal ‘planes’. Apart from dividing the abdomen into different regions for descriptive purposes, these planes are also of value in defining approximate vertebral levels and the positions of some relatively fixed intra-abdominal structures.

Horizontal planes

Several horizontal planes have been defined, but only the subcostal and transtubercular planes are in common clinical use.

The xiphisternal plane runs horizontally through the xiphoid processes at the level of the ninth thoracic vertebra. It demarcates the level of the cardiac plateau on the central part of the upper border of the liver.

The transpyloric plane lies midway between the suprasternal notch of the manubrium and the upper border of the pubic symphysis. It usually lies at the level of the body of the first lumbar vertebra near its lower border and meets the costal margins at the tips of the ninth costal cartilages, where a distinct ‘step’ may be felt at the costal margin. The linea semilunaris crosses the costal margin on the transpyloric plane. The hilum of both kidneys, the origin of the superior mesenteric artery, the termination of the spinal cord, the neck, adjacent body and head of the pancreas, and the confluence of the superior mesenteric and splenic veins as they form the hepatic portal vein may all lie in this plane. The pylorus may be found in the transpyloric plane, but is not a constant feature.

The subcostal plane is a line joining the lowest point of the costal margins, formed by the tenth costal cartilage on each side. It usually lies at the level of the body of the third lumbar vertebra, the origin of the inferior mesenteric artery from the aorta, and the third part of the duodenum, although this varies with posture.

The supracristal plane joins the highest point of the iliac crest on each side. It usually lies at the level of the body of the fourth lumbar vertebra, and marks the level of bifurcation of the abdominal aorta. On the posterior abdominal surface, it is a common level for the identification of the fourth lumbar vertebra, and is used to perform lumbar puncture at the L4–5 or L5–S1 intervertebral level, which is safely below the termination of the spinal cord.

The transtubercular plane joins the tubercles of the iliac crests and usually lies at the level of the body of the fifth lumbar vertebra near its upper border. It indicates, or is just above, the confluence of the common iliac veins and marks the origin of the inferior vena cava.

The interspinous plane joins the centres of the anterior superior spines of the iliac crests. It passes through either the lumbosacral disc, or the sacral promontory, or just below them, depending on the degree of lumbar lordosis, sacral inclination and curvature.

The plane of the pubic crest lies at the level of the inferior end of the sacrum or part of the coccyx, depending on the degree of lumbar lordosis, sacral inclination and curvature.

ANTERIOR ABDOMINAL WALL

Skeletal landmarks

The superior boundary of the anterior abdominal wall is formed by several clear landmarks (Fig. 60.11). In the midline superiorly lies the xiphoid process. From this point, the costal margins extend to either side from the seventh costal cartilage at the xiphisternal joint to the tip of the twelfth rib (the latter is often difficult to feel in the obese or if it is short). The lowest part of the costal margin lies in the midaxillary line and is formed by the inferior margin of the tenth costal cartilage. The tip of the lower border of the ninth costal cartilage can usually be defined as a distinct ‘step’ along the costal margin.

The inferior boundary of the anterior abdominal wall is formed, in order, by the iliac crest, which descends from the tubercle of the iliac crest to the anterior superior iliac spine; the inguinal ligament, which runs downwards and forwards to the pubic tubercle; and the pubic crest, which runs from the pubic tubercle laterally to the pubic symphysis in the midline. The pubic tubercle can be identified by direct palpation in thin individuals and can be detected, even in the obese, by palpation of the tendon of adductor longus, which runs up to its attachment on the pubis directly below the pubic tubercle. The tendon is best felt in tension with the hip flexed, abducted and externally rotated.

The posterolateral boundary is defined by the midaxillary line.

Soft tissue landmarks

INTRA-ABDOMINAL VISCERA

The surface markings of the intra-abdominal viscera are variable and depend on age, body habitus, nutritional state, phase of ventilation and body position. The use of radiological imaging of the abdominal viscera means that the location of viscera by surface markings is almost obsolete in modern clinical practice. The following descriptions are at best regarded as the most common or approximate markings in a healthy supine individual (Fig. 60.12).

image

Fig. 60.12 Intra-abdominal visceral landmarks. 1. Gallbladder. 2. Duodenum. 3. Appendix. 4. Root of small bowel mesentery.

(Photograph by permission from Lumley JSP 2002 Surface Anatomy, 3rd edn. Edinburgh: Churchill Livingstone.)

RETROPERITONEAL VISCERA

The surface projections of the retroperitoneal viscera are fairly reliable but have little clinical use since most interventions and procedures are guided by radiological imaging.

Abdominal aorta

The abdominal aorta starts just to the left of the midline, at the level of the body of the 12th thoracic vertebra. It continues downwards for 10 cm as a band 2 cm wide, and bifurcates at the level of the fourth lumbar vertebra (which is marked by the transtubercular plane), 1.5 cm below and to the left of the umbilicus. The pulsations of the aorta can be felt in a thin individual in the supine position by pressing firmly in the midline backwards onto the vertebral column. An easily palpable aorta in an obese person should raise the suspicion of an aneurysm, to be checked by radiological imaging.

CLINICAL PROCEDURES

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