THE BACK. 189 



its passage receiving blood from the flat plirenic or dia- 

 phragmatic venous sinuses. From the outer margin of 

 this tendinous portion muscular fibres radiate towards the 

 inner surfaces of the inferior extremities of the twelve 

 posterior ribs, or of their cartilages, where thej become 

 inserted in forming digitations with transversalis abdo- 

 minis, and towards the upper surface of the ensiform 

 cartilage. The indentation in the upper part of the tendon 

 is filled bj three muscular masses, two of which arise from 

 the left, the third from the right side of the tendon. Between 

 the right muscular mass and the central one is foramen 

 sinistrum, through which the oesophagus and the pneumo- 

 gastric nerves pass in their course from the thorax into the 

 abdomen; subsequently these two masses combine and 

 form a tendon, the right crus, which becomes attached to 

 the inferior vertebral ligament as far back as the fifth 

 lumbar vertebra; the third mass terminates in a much 

 shorter tendon is termed the left crus of the diaphragm, 

 and becomes inserted into the inferior vertebral ligament 

 below the body of the first lumbar vertebra. Between the 

 right and left crura is situated hiatus aorticus, which 

 admits the passage of the thoracic duct, posterior aorta, and 

 vena azygos, while passing through this opening the pos- 

 terior aorta gives off the phrenic artery which supplies the 

 crura of the muscle. That portion of the diaphragm ex- 

 tending from the spine to the internal surface of the last 

 rib passes over the anterior part of the inferior surface 

 of the psoas muscles, and above it also we see the gan- 

 gliated cord of the sympathetic and the greater and lesser 

 splanchnic nerves passing on either side. 



The chest is lined, and the viscera it contains covered 

 by the pleura, a serous membrane arranged in such a 

 manner as to form two sacs, one on each side of the 

 cavity, which lie together along the middle line so as to 

 form a septum extending in an antero-posterior direction, 

 consisting of two adjacent layers of pleura in some parts 

 in close apposition, in others separated by the interposi- 

 tion of important structures, but postero-inferiorly pre- 

 senting numerous perforations, whereby one pleural sac 

 is brought into communication with the other. This is 

 the mediastinum, and extends from the internal surface 

 of the two first ribs to the central line of the diaphragm. 

 ■Centrally it contains the heart, being reflected to form 



