ae 
THE MUSCLES OF THE THORAX. 395 
and from the arcuate igaments. The crura are two elongated fibro-muscular bundles 
which arise from the front of the bodies of the lumbar vertebre, on the right side from 
the first three, on the left side from the first two vertebrae. They are directed 
upwards, and passing in front of the aorta, decussate across the middle line in front 
of that vessel, the fibres of the right crus passing in front of those of the left crus. 
The fibres then encircle the cesophagus, forming an elliptical opening for its passage, 
and finally join the central tendon, after a second decussation in front of the 
gullet. 
The arcuate ligaments are five in number. 
The middle arcuate ligament is a fibrous arch connecting together the crura of 
the diaphragm in front of the aorta, and giving origin to fibres which join the crura 
as they decussate to encircle the gullet. 
The internal arcuate ligament is a thickening formed by the attachment of the 
psoas fascia to the body of the first lumbar vertebra internally and its transverse 
process externally. Stretching across the upper end of the psoas muscle, the 
(sophagus and its 
£ < opening 
, 
Foramen quadratum 
(for inferior cava) 
Internal arcuate 
ligament 
| f External arcuate 
WS pg jy ; i" i J ligament 
Middle arcuate liga- \y~ 
ment (in front of ~ 
aortic opening) 
\ 
T~__QuabRATUS LUMBORUM 
‘ MUSCLE 
~-PSOAS MUSCLE 
x 
Right crus of diaphragm Left crus of diaphragm 
Fic. 290.—THE DIAPHRAGM (from below). 
ligament gives origin to muscular fibres directed upwards on each side of the 
crura. 
The external arcuate ligament is the thickened upper end of the fascia over the 
quadratus lumborum (anterior layer of the lumbar fascia), and is attached inter- 
nally to the transverse process of the first lumbar vertebra, and externally to the 
last rib. It gives origin to another broad band of muscular fibres, separated 
from those arising from the internal arcuate ligament by an interval, and passing 
upwards to the central tendon of the diaphragm. 
From this extensive origin the muscular fibres of the diaphragm converge to 
an insertion into a large trilobed central tendon. Of its lobes the right one is the 
largest, the middle or anterior intermediate in size, and the left the smallest. It 
does not occupy the centre of the muscle, being placed nearer the front than the 
back. The fibres of the crura are consequently the longest ; those from the sternum 
the shortest. 
The diaphragm is pierced by numerous structures. The superior epigastric 
artery enters the sheath of the rectus abdominis between its sternal and costal 
