Diaphragm (Viewed From Above)
The
diaphragm is a curved musculotendinous septum separating the thoracic from the
abdominal cavity, forming the floor of the thoracic cavity with its convex upper
surface facing the thorax. The dome of the diaphragm on the right side is as
high as the fifth costal cartilage (varying with the phase of respiration) and
on the left is only slightly lower, so that some of the abdominal viscera are
covered by the thoracic cage.
The origin of the diaphragm is from the outlet of the
thorax and has three parts: sternal, costal, and lumbar.
The sternal origin is by two fleshy slips from the
back of the xiphoid process. The costal origin is by fleshy slips that
interdigitate with the slips of origin of the trans- versus abdominis muscle
and arise from the inner surfaces of the costal cartilages and adjacent parts
of the last six ribs on each side. The lumbar portion of the origin is
by a right and a left crus and right and left medial and lateral lumbocostal
arches (sometimes termed arcuate ligaments). The tendinous crura blend with the
anterior longitudinal ligament of the vertebral column and are attached to the
anterior surfaces of the lumbar vertebral bodies and related intervertebral discs to the first
three on the right and the first two on the left. The medial lumbocostal arch, a
thickening of the fascia covering the psoas major muscle, extends from the side
of the body of the first or second lumbar vertebra to the front of the
transverse process of the first (sometimes also the second) lumbar vertebra. The
lateral lumbocostal arch, passing across the quadratus lumborum muscle, extends
from the transverse process of the first lumbar vertebra to the tip and lower
border of the twelfth rib.
From the extensive origin just described, the fibers
converge to insert in a three-leafed central tendon. Contraction of the
muscular portion of the diaphragm pulls the central tendon downward, thus
increasing the volume of the thoracic cavity and bringing about inspiration.
The diaphragmatic nerve supply is by way of the right
and left phrenic nerves, which are branches of the right and left cervical
plexuses and receive their fibers primarily from the fourth cervical nerves,
with some contribution from the third and fifth cervical nerves.
Several structures pass between the thoracic and
abdominal cavities, mainly through apertures in the diaphragm.
The aortic aperture is at the level of the
twelfth thoracic vertebra situated between the diaphragm and the vertebra. It
transmits the aorta, azygos vein, and thoracic duct.
The esophageal aperture is located at the level
of the tenth thoracic vertebra in the fleshy part of the diaphragm. It transmits
the esophagus, the right and left vagus nerves, and small esophageal arteries
and veins.
The inferior vena caval aperture is situated at
the level of the disc between the eighth and ninth thoracic vertebrae at the
junction of the right and middle leaflets of the central tendon. It is traversed
by the inferior vena cava and some branches of the right phrenic nerve.
The right crus is pierced by the right greater and
lesser splanchnic nerves, and the left crus is pierced by the left greater and
lesser splanchnic nerves and the hemiazygos vein. The sympathetic trunks
usually do not pierce the diaphragm but pass behind the medial lumbocostal
arches.
The base of the fibrous pericardial sac is partially
blended with the middle leaflet of the central tendon of the diaphragm. The
diaphragmatic portions of the parietal pleura are closely blended with the
upper surfaces of the right and left portions of the diaphragm. Where the
diaphragmatic pleura reflects at a sharp angle to become the costal pleura, the
costodiaphragmatic recess or costophrenic sulcus is formed. Where the costal
pleura reflects to become pericardial pleura, the costomediastinal
recess is formed.
