Diaphragm
The dome-shaped roof of the abdominopelvic cavity
is formed by a musculoaponeurotic septum, the diaphragm, which also forms the
floor of the thoracic cavity and thus is the partition between these two cavities.
This unique muscular structure takes origin by its entire circumference from
the inner aspect of the lower margin of the thoracic cage. The muscular
fibers course superiorly and inward to insert in the margins of the diaphragm’s
central tendon. The sternal origin is, by way of a fairly short, fleshy
slip (a right and a left one), on the posterior aspect of the xiphoid process,
which, on its way to the anterior margin of the central tendon, does not ascend
nearly as much as do the fibers from the other two areas of origin. In fact,
depending on the position of the individual and the degree of contraction of
the diaphragm, the fibers from the sternal origin may even course inferiorly.
The costal origin in general arises from the inner surfaces of the costal
cartilages and the adjacent parts of the lower six ribs by fibers from each
that interdigitate with the origin of the transversus abdominis muscle. The
lumbar origin consists of a crus and a medial arcuate ligament and a lateral
arcuate ligament on each side. The crura begin as tendinous structures,
which attach to the anterior and lateral sides of the upper lumbar vertebrae
(one to three or four for the right and one to two or three for the left) and
related intervertebral disks, blending with the anterior longitudinal ligament.
The right crus is stouter as well as longer than the left and, as it
becomes muscular, usually splits to send a portion to the left of the esophageal
hiatus. The medial margins of the two crura converge to meet in the midline
to form an arch across the anterior aspect of the aorta, the median arcuate
ligament. The medial arcuate ligament is a tendinous arch that appears as a
thickening in the fascia over the superior part of the psoas major muscle, extending
from the side of the body of the second (or first) lumbar vertebra, where it
blends with the lateral margin of the corresponding crus to the end of the transverse
process of the first (or second) lumbar vertebra. The lateral
arcuate ligament is a thickening in the fascia that covers the quadratus
lumborum muscle and reaches from the end of the transverse process of the
1st (or 2nd) lumbar vertebra to the tip and lower margin of the 12th (or 11th)
rib.
The central tendon is a thin but strong and dense aponeurosis, closer to
the sternal origin than the costal and lumbar origins. It is shaped somewhat
like a thick and widely opened V, with slight indentations, which produce three
leaflets. The fibrous pericardium is blended with its superior surface.
Several openings (hiatuses) permit the passage of structures between the
thoracic and abdominal cavities.
The inferior vena cava passes through the caval opening at the
junction of the right and middle leaflets of the central tendon in the most
anterior and highest of the three large openings, being at the level of the
disc between T8 and T9. It often transmits a branch of the phrenic nerve. The esophageal
hiatus is in the muscular portion of the diaphragm just posterior to the
central tendon, a little to the left of the midline and about at the level of
T10, and transmits the esophagus as well as the anterior and posterior vagal
trunks to the abdomen.
The aortic hiatus (really a notch in the posterior margin of the
diaphragm) is at the level of T12 and transmits the thoracic duct and azygos
vein in addition to the aorta.
The greater and lesser
splanchnic nerves pierce the crura, and, in addition, the left crus is
pierced by the hemiazygos vein.
Intervals at the origin where muscle is replaced by areolar connective tissue occur at the
sternocostal tri-angle and, with great variation, at the lateral arcuate ligaments.
