940 A MANUAL OF ANATOMY 



as follows : in front, the sternum ; behind, the bodies of the thoracic 

 vertebrae; and, on either side, the corresponding pleural sac. The 

 mediastinal space contains structures so numerous as to necessitate 

 its subdivision into four parts — superior, anterior, middle, and 

 posterior. 



The superior mediastinum is situated above the pericardium, and 

 its boundaries are as follows : in front, the deep surface of the manu- 

 brium sterni, with the origins of the sterno-hyoid and sterno-thyroid 

 muscles ; behind, the bodies of the upper four thoracic vertebrae 

 and the lower portions of the longus colli muscles ; above, an imagi- 

 nary plane corresponding to the superior aperture of the thorax ; 

 below, an imaginary plane passing from the lower border of the 

 manubrium sterni to the lower border of the body of the fourth 

 thoracic vertebra ; and, laterally, the mediastinal pleura of each 

 side as it extends from the deep surface of the manubrium sterni 

 to the vertebral column. 



Contents. — ^These are as follows : (i) the arch of the aorta ; (2) the 

 innominate artery, and the thoracic portions of the left common 

 carotid and left subclavian arteries ; (3) the right and left innominate 

 veins, and the upper half, or extrapericardial portion, of the superior 

 vena cava ; (4) the phrenic, pneumogastric, left recurrent laryngeal, 

 and cardiac nerves ; (5) the trachea, oesophagus, and thoracic duct ; 

 (6) the superior mediastinal glands ; and (7) the remains of the 

 thymus body. 



The anterior mediastinum is situated behind the body of the 

 sternum, and its boundaries are as follows : in front, the deep surface 

 of the body of the sternum, and the left triangularis sterni muscle ; 

 behind, the pericardium ; and, laterally, the mediastinal pleura of 

 each side as it extends from the deep surface of the body of the 

 sternum to the lateral aspects of the pericardium. Its direction is 

 downwards and to the left. At its upper part there is no interspace, 

 because the two pleural sacs come into contact behind the first 

 piece of the body of the sternum. Below this level, however, the 

 left pleural sac diverges from its fellow, so as to leave an interspace. 



Contents. — ^These are the anterior mediastinal glands, some 

 lymphatic vessels, the anterior mediastinal branches of the internal 

 mammary artery, and areolar tissue. 



Anterior Mediastinal Glands. — These glands are situated within 

 the areolar tissue of the lower part of the anterior mediastinum, 

 between the lower part of the body of the sternum and the front : 

 of the pericardium. 



They receive their afferent vessels from (i) the antero-mesial 

 portion of the diaphragm, corresponding to the ensiform process 

 of the sternum, (2) the supero-anterior surface of the liver, on 

 either side of the falciform ligament, (3) the anterior part of the 

 pericardium, (4) the anterior part of the mediastinal pleura, and 

 (5) the lower sternal, or internal mammary, glands. Their efferent 

 vessels ascend and open into the superior mediastinal or cardiac 

 glands. 



