THE MUSCLES OF THE TRUNK. 



of the posterior margin of the tendinous centre. It frequently happens that fleshy 

 fibres spring only from the inner portion of the external arched ligament, and in 

 that case a triangular interval is left between the vertebral and costal portions of the 

 muscle, through which the areolai\tissue of the thoracic cavity becomes continuous 

 with that of the abdomen. 



The costal portion of the muscle consists of a series of serrated slips arising from 

 the cartilages of the six lower ribs, and interdigitating with the attachments of the 

 transversalis abdominis muscle (fig. 288). They sometimes arise also in part from 

 the osseous ribs. The sternal portion is a narrow slip, sometimes divided into two, 

 separated on each side from the costal portion by an interval which gives passage to 

 the epigastric division of the internal mammary vessels together with some lymphatics, 

 and in which the lining membranes of the thorax and abdomen are separated only 

 by a small quantity of loose connective tissue. The sternal fibres of the diaphragm 

 are much the shortest ; the fibres from the eighth and ninth rib-cartilages are the 

 longest. The fibres of the sternal and costal slips, after being united, rise in an 

 arched and converging manner to be inserted into the anterior and external margins 

 of the central tendon. 



The central tendon trefoil, or cordiform tendon is a strong aponeurosis, forming 

 the central and highest part of the diaphragm. It is elongated from side to side, 

 convex in front and concave behind, and consists of three lobes or alee, partly separated 

 by indentations. The right lobe is the largest ; and the left, which is elongated and 

 narrow, is the smallest of the three. The central tendon is surrounded on every side 

 by the muscular portion of the diaphragm, the fibres of which are directly continuous 

 with those of the tendon. The tendinous fibres cross one another, and are inter- 

 woven in various directions. 



Foramina. There are in the diaphragm three large perforations for the passage 

 respectively of the aorta, the oesophagus, and the vena cava, besides some smaller 

 holes or fissures which are less regular. a. The foramen for the aorta (hiatus 

 aorticus), placed in front of the first lumbar vertebra, is bounded by tendinous 

 fibres of the crura as already described. Besides the aorta, this opening transmits 

 the thoracic duct, and generally also the large azygos vein. b. The foramen for the 

 oesophagus is opposite the tenth dorsal vertebra, and lies farther forward than, as 

 well as a little to the left of the aortic opening, from which it is separated by the 

 decussating fibres of the crura. It is oval in form, and is generally entirely sur- 

 rounded by muscular fibres ; in some rare cases, however, a small part in front is 

 formed by the margin of the central tendon. Through it pass also the pneumo- 

 gastric nerves and oesophageal branches of the coronary artery. Its margin is con- 

 nected to the structures passing through only by loose areolar tissue, c. The opening 

 for the vena cava (foramen quadratum) is placed in the highest part of the diaphragm 

 about the level of the disc between the eighth and ninth dorsal vertebrae, in the 

 tendinous centre at the junction of the right and middle lobes, posteriorly. Its 

 form is somewhat quadrangular ; and it is bounded by fasciculi of tendinous fibres 

 running parallel with its sides, and firmly attached to the wall of the vessel. Besides 

 the foregoing large foramina there are small perforations through the crura for the 

 splanchnic nerves on both sides, and for the small azygos vein on the left side ; 

 while the large azygos vein often takes its course through the right crus. The cord 

 of the sympathetic nerve either perforates the outer part of the crus or passes under 

 the internal arched ligament ; and the last dorsal nerve descends on the quadratus 

 lumborum beneath the external arched liament. 





Form, and relations. The upper or thoracic surface of the diaphragm is highly arched. 

 Its posterior and lateral fibres, ascending from their connection with the lower margin of the 

 thorax, are for a considerable extent placed close to the ribs, the lungs, especially in their 



