1188 



THE ORGANS OF VOICE AND RESPIRATION 



upper portion, above the upper level of the pericardium, which is named the 

 superior mediastinum; and a lower portion, below the upper level of the peri- 

 cardium. This lower portion is again subdivided into three that part which 

 contains the pericardium and its contents, the middle mediastinum; that part 

 which is in front of the pericardium, the anterior mediastinum; and that part 

 which is behind the pericardium, the posterior mediastinum. 



The superior mediastinum (Fig. 901) is that portion of the interpleural space 

 which lies between the manubrium sterni in front and the upper thoracic vertebrae 

 behind. It is bounded below by a plane passing backward from the junction 

 of the manubrium and gladiolus sterni to the lower part of the body of the fourth 

 thoracic vertebra, and laterally by the lungs and pleune. It contains the origins 

 of the Sternohyoid and Sternothyroid muscles and the lower ends of the Longus 

 colli muscles; the arch of the aorta; the innominate, the thoracic portion of the 

 left carotid and subclavian arteries; the upper half of the superior vena cava and 



Left Innom- Left Carotid Thymus 



in nt K Vein. Ar1.p/rii dlnnd 



inate Vein. Artery 



Vagus 



Nerve. 

 / 



Vagus Nerve. 



Vertebral 



Artery 



Left Sublcavian 



Artery. 



Oesophagus.- 



^Internal Mammary 

 Artery. 



Right Innom- 



^- inate Vein. 



1st Rib. 



Trachea. 



2nd Rib. 



4 e* 



FIG. 901. Transverse section through the second thoracic vertebra. (Braune.) 



the innominate veins, and the left superior intercostal vein; the vagus, cardiac, 

 phrenic, and left recurrent laryngeal nerves; the trachea, oesophagus, and thoracic 

 duct; the remains of the thymus gland and some lymph nodes. 



The anterior mediastinum (Fig. 897) is bounded in front by the sternum, 

 laterally by the pleurae, and behind by the pericardium. It is narrow above, 

 but widens out a little below, and, owing to the oblique course taken by the left 

 pleura, it is directed from above obliquely downward and to the left. Its anterior 

 wall is formed by the left Triangularis sterni muscle and the fifth, sixth, and seventh 

 left costal cartilages. It contains a quantity of loose areolar tissue, some lym- 

 phatic vessels which ascend from the convex surface of the liver, two or three 

 lymph nodes (anterior mediastinal glands), and the small mediastinal branches 

 of the internal mammary artery. 



The middle mediastinum (Fig. 897) is the broadest part of the interpleural 

 space. It contains the heart enclosed in the pericardium, the ascending aorta, 

 the lower half of the superior vena cava, with the vena azygos major opening into 



