THE MEDIASTINA. 445 



by a plane extending from the lower border of the body of the 

 fourth dorsal vertebra to the junction of the manubrium with the 

 body of the sternum. It is limited in front by the manubrium 

 with the origins of the sterno-hyoid and sterno-thyroid muscles, 

 and behind by the upper four dorsal vertebrae and the lower ends 

 of the longi colli muscles. Between the pleurae in this part there 

 are found, proceeding from before backwards, the following objects : and 

 the remains of the thymus gland, several lymphatic glands, the 

 innominate veins (fig. 162), and the upper half of the superior 

 cava, the phrenic and pneumo-gastric nerves, the arch of the aorta 

 with its three large branches, innominate, left carotid and left 

 subclavian, the trachea, oesophagus and thoracic duct, and the left 

 recurrent laryngeal nerve. 



The anterior mediastinum is the space in front of the pericardium J^jjjjj 

 between the pleurae, and is very narrow in its upper half, since the num is the 

 two pleurae meet behind the sternum from the level of the second smaUest : 

 to the fourth costal cartilages. Below the latter spot the left pleura 

 inclines away from the middle line, and is separated from its fellow 

 by an interval in which the pericardium comes into contact 

 with the sternum and the left triangularis sterni muscle. This contents, 

 mediastinum contains only some areolar tissue, with a few small 

 lymphatic glands. In some bodies the left pleura is continued 

 behind the sternum nearly as far as the diaphragm. 



The middle mediastinum is the largest part of the central space, and Middle me- 

 includes the pericardium with the contained heart and great vessels, contents, 

 viz., the ascending aorta, the trunk of the pulmonary artery, and 

 the lower half of the superior vena cava ; also the phrenic nerves, 

 the roots of the lungs with the bronchial lymphatic glands, and, on 

 the right side, the arch of the azygos vein. 



The posterior mediastinum is the portion between the pericardium Posterior 

 and the spine ; and the interpleural space is here larger than in Snulnl 

 front of the heart. Its extent and contents will be shown later, 

 but it may be here said that enclosed between the serous layers boundaries, 

 of the posterior mediastinum (fig. 174, p. 481) are the descending and con- 

 thoracic aorta, the azygos veins, the oesophagus with the pneumo- tents * 

 gastric nerves, and the thoracic duct, as well as a set of lymphatic 

 glands. 



Dissection. The pleurae and the fat are now to be cleaned from Clean peri- 

 the sides of the pericardium. 



The root of the lung is to be dissected oat by taking away the the root of 

 pleura and the areolar tissue from the front and back without th 

 injuring its component vessels. To clean the back of the root, the 

 lung should be thrown forwards to the opposite side of the chest. 

 In this dissection the phrenic nerve and artery will be found in Trace the 

 front of the root, together with a few small anterior pulmonary ne 

 nerves ; the last are best seen on the left side. Behind the root of 

 the lung is the vagus nerve, dividing into branches ; and arching 

 above the right one is the large azygos vein. and azygos 



For the present, the arch of the aorta and the small nerves on it vein< 

 may be left untouched. 



