(b) to
the costal process of the first lumbar vertebra in front of the
quadratus lumborum muscle (o), the _internal_ and _middle_ are
continuations of the _crura_ which rise from the ventro-lateral aspects
of the bodies of the lumbar vertebrae, the right (e) coming from three,
the left (f) from two. On reaching the level of the twelfth thoracic
vertebra each crus spreads out into a fan-shaped mass of fibres, of
which the innermost join their fellows from the opposite crus, in front
of the aortic opening (k), to form the _middle arcuate ligament_; the
outer ones (g) arch in front of the psoas muscle (n) to the tip of the
costal process of the first lumbar vertebra to form the _internal
arcuate ligament_, while the intermediate ones pass to the central
tendon. There are three large openings in the diaphragm; the _aortic_
(k) is behind the middle arcuate ligament and transmits the aorta, the
vena azygos major, and the thoracic duct. In the right leaflet is an
opening (sometimes called the _hiatus quadratus_) for the inferior vena
cava and a branch of the right phrenic nerve (m), while in front and a
little to the left of the aortic opening is one for the oesophagus and
the two pneumogastric nerves (l), the left being in front and the right
behind. The fleshy fibres on each side of this opening act as a
sphincter. Passing between the xiphoid and costal origins in front are
the superior epigastric arteries, while the other terminal branches of
the internal mammaries, the musculo-phrenics, pass through between two
costal origins.

[Illustration: FIG. 1.--Abdominal Surface of the Diaphragm.]

Through the crura pass the splanchnic nerves, and in addition to these
the left crus is pierced by the vena azygos minor. The sympathetic
nerves usually enter the abdomen behind the internal arcuate ligaments.
The phrenic nerves, which are the main supply of the diaphragm, divide
before reaching the muscle and pierce it in a number of places to enter
its abdominal surface, but some of the lower intercostal nerves assist
in the supply. The last thoracic or subcostal nerves pass behind the
external arcuate ligament.

For the action of the diaphragm see RESPIRATORY SYSTEM.

_Embryology._--The diaphragm is at first developed in the neck region
of the embryo, and this accounts for the phrenic nerves, which supply
it, rising from the fourth and fifth cervical. From the mesoderm on
the caudal side of the pericardium is developed the