46 THORAX 



to the heart, and the phrenic nerves with their accompanying 

 vessels. These sections of the mediastinum and their con- 

 tents must now be examined in detail. 



Dissection. The remains of the anterior part of the mediastinal pleura 

 must be divided longitudinally immediately posterior to the sternum, from 

 the lower end of the thorax to the apices of the pleural sacs. The sternal 

 extremities of the first ribs must be then cut through, close to the manu- 

 brium sterni, and, at the same time, the sternal heads of the sterno-mastoicl 

 muscles must be separated from the manubrium, if that has not already 

 been done by the dissector of the head and neck. After the sterno- 

 mastoicl muscles and the first ribs are divided, the sterno-hyoid and sterno- 

 thyreoid muscles must be cut through transversely, as close to the upper 

 margin of the manubrium as possible. Next, the body of the sternum must 

 be separated from the xiphoid process and the tips of the seventh costal 

 cartilages. The sternum with the attached costal cartilages may then be 

 removed and placed aside, but it must be carefully preserved for future use. 



When the sternum is removed the mediastinum is exposed from the 

 front. As seen from the front, the superior mediastinum, which lies posterior 

 to the manubrium, is a relatively wide triangular area, with its apex below. 

 The anterior mediastinum, on the other hand, is merely a narrow cleft 

 between the adjacent anterior margins of the pleural sacs, except opposite 

 the anterior end of the left fifth costal cartilage where the left pleural sac 

 deviates slightly to the left and the anterior mediastinum becomes slightly 

 wider (Fig. 20). 



The anterior parts of both the superior and the anterior mediastina are 

 occupied by areolar tissue in which, as far down as the third or fourth 

 costal cartilages, remains of the thymus gland may be found. 



Thymus. The thymus gland is a bilobed organ, developed 

 from the third visceral clefts. It is well developed in the 

 foetus and in the child until the end of the second year. 

 Then it frequently undergoes atrophy, but it may persist even 

 until old age. 



Dissection. All the remains of the mediastinal pleura and the thymus gland 

 should now be taken away, and the anterior surface of the pericardium and 

 the contents of the superior mediastinum should be thoroughly cleaned. 

 When this has been done the right and left innominate veins and their 

 tributaries will be exposed. The innominate veins should be traced to 

 their union with the superior vena cava. To the left of the superior vena 

 cava and below the left innominate vein lie the upper part of the ascending 

 portion of the aorta, and the anterior part of the aortic arch. When these 

 contents of the upper part of the mediastinum have been thoroughly cleaned, 

 the various structures found in the mediastinum must be studied in detail. 



Venae Anonymae. -The innominate vein of each side is 

 formed posterior to the sternal end of the corresponding clavicle 

 by the union of the internal jugular and subclavian veins 

 of the same side, and it ends, at the lower border of the 

 right first costal cartilage, by uniting with its fellow of the 

 opposite side to form the superior vena cava. 



The right innominate vein is short and its course is 



