496 MEDIASTINUM ABDOMEN. 



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 medi- 

 astinum, but have 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 Jlnterior 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 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 in- 

 ternal mammary vessels of the left side. 



The Middle mediastinum contains the heart enclosed in its pericardium ; 

 the ascending aorta ; the superior vena cava ; the bifurcation 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 mus- 

 cles ; 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 of refer- 

 ence 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 inferior convexities of the ribs, the other 

 with the highest points of the crests of the ilia, the abdomen will be divided 

 into three zones. Again, if a perpendicular line be drawn at each side, 

 from the cartilage'of the eighth rib to the middle of Poupart's ligament, 

 the three primary zones will each be subdivided into three compartments 

 or regions, a middle and two lateral. 



The middle region of the upper zone being immediately over the sh.all 

 end of the stomach, is called epigastric (M ya<r<nip, over the stomach). 

 The two lateral regions being under the cartilages of the ribs are called 

 lypochondriac (iko XQ'V^OI, under the cartilages). The middle region of 

 the middle zone is the umbilical ; the two lateral, the lumbar. The middle 

 region of the inferior zone is the hypogastric (u^o ya<r<n^, below the sto- 

 mach) ; and the two lateral, the iliac. In addition to these divisions we 



