The diaphragm is a musculotendinous partition separating the thoracic and abdominal cavities. Its periphery consists of skeletal muscle fibres, which merge centrally with an aponeurotic tendon (Fig. 4.104). The diaphragm has a pronounced convexity towards the thorax and has two domes or cupolas, the right usually lying at a higher level than the left (Figs 4.105 & 4.106).
Peripherally, the diaphragmatic muscle is attached to the sternum, the costal margin and the vertebral column. The sternal attachment is by two small slips to the posterior surface of the xiphisternum. The costal attachment is to the inner surfaces of the lower ribs (usually 7–12) and costal cartilages (Fig. 4.105) by slips that interdigitate with those of transversus abdominis.
The diaphragm attaches to the vertebral column by two crura (pillars), one on each side of the abdominal aorta (Fig. 4.104). Both crura are anchored to the sides of the upper two lumbar vertebral bodies while the longer right crus is also attached to the third lumbar vertebra. The crura are linked in front of the aorta by the median arcuate ligament (Fig. 4.104), the fibres interdigitating as they ascend towards the central tendon. Lateral to each crus the diaphragm attaches to the transverse process of the first lumbar vertebra by the medial arcuate ligament (lumbocostal arch) and to the twelfth rib by the lateral arcuate ligament.
A triangular gap, the lumbocostal triangle (Fig. 4.104), often exists between the fibres attaching to the last rib and those arising from the vertebral column. This is a site where abdominal organs may herniate into the thorax.
The muscle fibres of the diaphragm converge on the margins of the central tendon, a V-shaped area of dense fibrous tissue with its apex directed towards the xiphisternum and its lateral parts running backwards into the domes (Fig. 4.104). The central tendon gives attachment to the fibrous pericardium and is pierced by the inferior vena cava (Figs 4.104 & 4.106).
Structures passing between the thorax and abdomen
Apertures in the diaphragm transmit the inferior vena cava and the oesophagus. The opening for the inferior vena cava (caval opening) lies to the right of the midline, and the oesophageal opening (hiatus) is slightly to the left (Fig. 4.104). During quiet breathing these openings lie at the levels of the eighth and tenth thoracic vertebrae, respectively. The caval opening pierces the central tendon and transmits the right phrenic nerve as well as the vena cava. The oesophageal opening, which also transmits the vagal trunks and branches of the left gastric vessels, is surrounded by muscle fibres of the right crus, which can constrict the oesophagus (Fig. 4.104). Weakness of this musculature may permit regurgitation of gastric contents or herniation of part of the stomach into the thorax (hiatus hernia). Patients may have difficulty with swallowing (dysphagia). The left phrenic nerve pierces the left dome adjacent to the apex of the heart, while on each side the thoracic splanchnic nerves pass through the crura to reach the coeliac plexus. The left crus may also be pierced by the hemiazygos vein.
The aorta enters the abdomen by descending behind rather than through the diaphragm and is accompanied by the thoracic duct and azygos vein (Fig. 4.107). The three vessels pass behind the median arcuate ligament and in front of the twelfth thoracic vertebral body. The subcostal nerves and vessels enter the abdomen behind the lateral arcuate ligaments anterior to quadratus lumborum, while the sympathetic trunks descend behind the medial arcuate ligaments anterior to psoas major (Fig. 4.104). Close to the xiphisternum, the superior epigastric vessels (branches of the internal thoracic vessels) pass between the sternal and costal slips of the diaphragm to enter the rectus sheath. Around the periphery of the diaphragm, intercostal nerves and vessels pass between the muscular slips to leave the lower intercostal spaces and reach the abdominal wall.
The diaphragm is an important muscle of inspiration and also assists the muscles of the abdominal walls and pelvic floor in raising the pressure within the abdomen and pelvis. Thus, the diaphragm contracts during acts of lifting and straining (e.g. defecation and childbirth).
Its shape and position vary with body position and the phase of ventilation. During full inspiration, the central tendon descends to approximately the level of the tenth thoracic vertebra. This descent, which is enhanced by an upright body posture, enlarges the thoracic cavity. When the diaphragm relaxes during expiration, its central tendon is pushed superiorly by intra-abdominal pressure, compressing the thoracic contents. With the body recumbent or head downwards, this upwards displacement is accentuated by the weight of the abdominal organs.
The right and left phrenic nerves provide the motor and main sensory supply to the diaphragm. The phrenic nerves arise in the neck, from the third, fourth and fifth cervical nerves (p. 236). Each nerve descends through the thorax (p. 60) and divides into terminal branches that pierce the diaphragm and innervate it from the abdominal surface.
Each phrenic nerve provides the motor supply to its own half of the diaphragm. In addition, each phrenic nerve carries sensory fibres from the pericardium and from pleura and peritoneum covering the central portion of the diaphragm. Stimulation of these fibres, for example by blood from a ruptured spleen, may produce pain referred to the shoulder region because the skin over the shoulder is also supplied by the fourth cervical segment of the spinal cord. By contrast, the pleura and peritoneum clothing the peripheral parts of the diaphragm are innervated by sensory branches of the lower intercostal nerves.
The major blood supply is provided by the inferior phrenic arteries (Fig. 4.104), which are usually direct branches of the aorta (Fig. 4.92). The corresponding veins drain into the inferior vena cava. Also, the musculophrenic vessels (terminal branches of the internal thoracic vessels; Fig. 2.17) supply the periphery of the diaphragm.
The inferior surface of the diaphragm is in contact with abdominal organs including the liver, kidneys, spleen and stomach. Its thoracic surface is related to the heart and lungs and their associated pericardium and pleura (Figs 4.105 & 4.106). Upward and down-ward excursions of the diaphragm cause corresponding movements of all the organs related to it. During the later stages of expiration, the periphery of the diaphragm comes into contact with the chest wall as the costodiaphragmatic recesses deepen (Fig. 4.105).