DIAPHRAGM. 21? 



substance of the last dorsal and first lumbar vertebra, and terminates in a 

 long slender tendon which expands inferiorly and is inserted into the ilio- 

 pectineal line and eminence. The tendon is continuous by its outer bor- 

 der with the iliac fascia. 



Relations. It rests upon the psoas magnus, and is covered in by the 

 peritoneum ; superiorly it passes beneath the ligamentum arcuatum of the 

 diaphragm. It is occasionally wanting. 



DIAPHRAGM. To obtain a good view of this important inspiratory 

 muscle, the peritoneum should be dissected from its under surface. It is 

 the muscular septum between the thorax and abdomen, and is composed 

 of two portions, a greater and a lesser muscle. The greater muscle arises 

 from the ensiform cartilage ; from the inner surfaces of the six inferior 

 ribs, indigitating with the transversalis ; and from the ligamentum arcua- 

 tum externum and internum. From these points, which form the internal 

 circumference of the trunk, the fibres converge and are inserted into the 

 central tendon. 



The ligamentum arcuatum externum is the upper border of the anterior 

 lamella of the aponeurosis of the transversalis : it arches across the origin 

 of the quadratus lumborum muscle, and is attached by one extremity to 

 the base of the transverse process of the first lumbar vertebra, and by the 

 other to the apex and lower margin of the last rib. 



The ligamentum arcuatum internum, or proprium, is a tendinous arch 

 thrown across the psoas magnus muscle as it emerges from the chest. It 

 is attached by one extremity to the base of the transverse process of the 

 first lumbar vertebra, and by the other is continuous with the tendon of 

 the lesser muscle opposite the body of the second. 



The tendinous centre of the diaphragm is shaped like a trefoil leaf, of 

 which the central leaflet points to the ensiform cartilage, and is the largest ; 

 the lateral leaflets, right and left, occupy the corresponding portions of the 

 muscle ; the right being the larger and more rounded, and the left smaller 

 and lengthened in its form. 



Between the sides of the ensiform cartilage and the cartilages of the 

 adjoining ribs, is a small triangular space where the muscular fibres of the 

 diaphragm are deficient. This space is closed only by peritoneum on the 

 side of the abdomen, and by pleura within the chest. It is therefore a 

 weak point, and a portion of the contents of the abdomen might, by vio- 

 lent exertion, be forced through it, producing phrenic, or diaphragmatic 

 hernia. 



The lesser muscle of the diaphragm takes its origin from the bodies of 

 the lumbar vertebrae by two tendons. The right, larger and longer than 

 the left, arises from the anterior surface of the bodies of the second, third, 

 and fourth vertebrae ; and the left from the side of the second and third. 

 The tendons form two large fleshy bellies (crura), which ascend to be in- 

 serted into the central tendon. The inner fasciculi of the two crura cross 

 each other in front of the aorta, and again diverge to surround the oeso- 

 phagus, so as to present the appearance of a figure of eight. The ante 

 rior fasciculus of the decussation is formed by the right crus. 



The openings in tJie diaphragm are three : one, quadrilateral, in the 

 tendinous centre, at the union of the right and middle leaflets, for the 

 passage of the inferior vena cava ; a muscular opening of an elliptic shape 

 formed by the two crura, for the transmission of the esophagus and pneu- 

 mogastric nerves ; and a third, the aortic, which is formed by a tendinous 



