420 THE MUSCLES AND FASCIA 



lumbar vertebrae by two crura; and from aponeurotic arches named the arcuate 

 ligaments (pars lumbalis). 



The crura are situated on the bodies of the lumbar vertebrae, on each side of 

 the aorta. The crura, at their origin, are tendinous in structure; the right crus, 

 larger and longer than the left, arises from the anterior surface of the bodies and 

 articular disks of the four upper lumbar vertebrae; the left crus arises from the 

 three upper lumbar vertebra?; both blend with the anterior common ligament of 

 the vertebral column. 



The tendinous portion of each crus passes forward and inward to meet the 

 corresponding portion of the opposite side, thus forming the tendinous arch known 

 as the middle arcuate ligament. From this arch diverging muscle fibres arise, 

 the outermost portion of which is directed upward and outward to the central 

 tendon; the innermost portion decussate in front of the aorta, diverge in order 

 to surround the oesophagus, and likewise end in the central tendon. The fibres 

 derived from the right crus are the more numerous and pass in front of those 

 derived from the left. 



Each crus, in its tendinous portion, is perforated by the great and lesser 

 splanchnic nerves and sometimes the ascending lumbar radicle of the azygos 

 veins. 



The internal arcuate ligament (arcus lumbocostalis medialis} is a tendinous arch 

 in the fascia covering the upper part of the Psoas magnus muscle, on each side 

 of the vertebral column. This tendinous arch is connected mesad to the body 

 of the first (or second) lumbar vertebrae, laterad to the front of the tip of the 

 transverse process of the first (or second) lumbar .vertebrae. 



The external arcuate ligament (arcus lumbocostalis lateralis) is the thickened 

 upper margin of the anterior lamella of the lumbar fascia ; it arches across the 

 upper part of the Quadratus lumborum from the front of the transverse process 

 of the first (or second) lumbar vertebra to the apex and lower margin of the 

 twelfth rib. 



The fibres of the Diaphragm derived from these sources vary in length; those 

 arising from the ensiform cartilage are short and occasionally aponeurotic, con- 

 tinuous with the posterior layer of the Rectus abdominis sheath. Those arising 

 from the arcuate ligaments, and more especially those from the cartilages of the 

 ribs at the side of the thorax, are longer, describe w T ell-marked curves as they ascend, 

 and finally converge to be inserted into the margin of the central tendon. 



The central tendon of the Diaphragm (centrum tendineum) is a thin but strong 

 tendinous aponeurosis, situated at the centre of the vault formed by the muscle, 

 immediately below the pericardium, with which it is partly blended. It is shaped 

 somewhat like a trefoil leaf, consisting of three divisions, or leaflets, separated 

 from one another by slight indentations. The right leaflet is the largest; the 

 middle one, directed toward the ensiform cartilage, the next in size; and the 

 left, the smallest. In structure, the tendon is composed of several planes of fibres 

 which intersect one another at various angles, and unite into straight or curved 

 bundles an arrangement which affords it additional strength. 



The openings in the Diaphragm are three large and several smaller apertures. 

 The former are the aortic, the ossophageal, and the opening for the inferior vena 

 cava. 



The aortic opening (hiatus aorticus) is the lowest and the most posterior of the 

 three large apertures connected with this muscle, being at the level of the first 

 lumbar vertebra. It is situated slightly to the left of the middle line, immediately 

 in front of the bodies of the vertebrae; and is, therefore, behind the Diaphragm, not 

 in it. It is an osseoaponeurotic aperture, formed by a tendinous arch (middle 

 arcuate ligament) thrown across the front of the bodies of the vertebrae, from the 

 crus on one side to that on the other, and it transmits the aorta, the vena azygos 



