DIAPHRAGM
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).
Musculature
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.
Central tendon
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.
Movements
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.
Blood supply
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.
Relations
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).
