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
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