MEDIASTINUM. ABDOMEN. 553 



membrane which is in relation with the lung is called pleura pulmo- 

 nalis, and that in contact with the parietes, pleura costalis. The re- 

 flected portion, besides forming the internal lining to the ribs and in- 

 tercostal muscles, also covers the diaphragm and the thoracic surface 

 of the vessels at the root of the neck. 



The pleura must be dissected from off the root of the lung, to see the 

 vessels by which it is formed and the pulmonary plexuses. 



MEDIASTINUM. 



The approximation of the two reflected pleurae in the middle line 

 of the thorax forms a septum, which divides the chest into the two 

 pulmonary cavities. This is the mediastinum. The two pleurae are 

 not, however, in contact with each other at the middle line in the 

 formation of the mediastinum, but leave a space between them which 

 contains all the viscera of the chest with the exception of the lungs. 

 The mediastinum is divided into the anterior, middle, and posterior. 



The Anterior mediastinum is a triangular space, bounded in front by 

 the sternum, and on each side by the pleura. It contains a quantity 

 of loose areolar tissue, in which are found some lymphatic glands and 

 vessels passing upwards from the liver ; the remains of the thymus 

 gland, the origins of the sterno-hyoid, sterno-thyroid, and triangularis 

 sterni muscles, and the internal mammary vessels of the left side. 



The Middle mediastinum contains the heart enclosed in its peri- 

 cardium ; the ascending aorta ; the superior vena cava ; the bifurca- 

 tion of the trachea ; the pulmonary arteries and veins ; and the phrenic 

 nerves. 



The Posterior mediastinum is bounded behind by the vertebral 

 column, in front by the pericardium, and on each side by the pleura. 

 It contains the descending aorta ; the greater and lesser azygos veins, 

 the superior intercostal vein ; the thoracic duct ; the oesophagus and 

 pneumogastric nerves ; and the great splanchnic nerves. 



ABDOMEN. 



The abdomen is the inferior cavity of the trunk of the body ; it is 

 bounded in front and at the sides by the lower ribs and abdominal 

 muscles ; behind, by the vertebral column and abdominal muscles ; 

 above, by the diaphragm ; and, below, by the pelvis : and contains 

 the alimentary canal, the organs subservient to digestion, viz. the liver, 

 pancreas, and spleen ; and the organs of excretion, the kidneys, with 

 the supra-renal capsules. 



Regions. For convenience of description of the viscera, and for re- 

 ference to the morbid affections of this cavity, the abdomen is divided 

 into certain districts or regions. Thus, if two transverse lines be 

 carried around the body, the one parallel with the convexities of the 



