1 54 The Thorax 



front of the pericardium. It inclines, therefore, to the left. In front 

 of it are the sternum and the adjoining parts of the fifth, sixth, and 

 seventh cartilages. The triangularis sterni covers the anterior 

 boundary of the space and shuts out the internal mammary vessels. 

 It contains lymphatics ascending from the liver, and some small 

 lymphatic glands which are associated with the inner part of the 

 mamma (p. 203). 



Abscess in the root of the neck readily finds its way into the 

 anterior mediastinum by following the trachea or the sterno-hyoid and 

 thyroid muscles. 



The middle mediastinum contains the heart and pericardium, and 

 the arterial and venous trunks which the pericardium encloses ; the 

 roots of the lungs, and the various structures associated therewith ; 

 the vena azygos turning over the root of the right lung, and the 

 phrenic nerves. 



The posterior mediastinum is limited by that part of the spine 

 which is behind the pericardium and the roots of the lungs. In it are 

 the third part of the arch and the thoracic aorta ; the oesophagus with 

 the vagi ; the azygos veins, the thoracic duct, and some lymphatic 

 glands, and the sympathetic chains with their splanchnic offshoots. 



The superior mediastinum is defined by drawing an imaginary 

 horizontal plane from the transverse sternal ridge (p. 149) to the lower 

 border of the fourth dorsal vertebra. It contains the origin of the sterno- 

 hyoid and thyroid muscles, the remains of the thymus gland, trachea, 

 oesophagus, left recurrent laryngeal nerve, thoracic duct ; the trans- 

 verse part of the aortic arch with the origin of the innominate, left 

 carotid, and left subclavian arteries ; the innominate veins and the be- 

 ginning of the superior cava ; the phrenic, pneumogastric, and cardiac 

 nerves, and lymphatic glands. 



The thymus (dvpos, soul, life) is a ductless gland lying between 

 the manubrium and the aortic arch ; in early life it ascends into the 

 root of the neck, in front of the trachea ; below it reaches to the peri- 

 cardium. It increases in bulk to the end of the second year, at which 

 time, as regards size, it is an important anatomical structure. (See 

 wood-cut on next page.) 



Its arteries are derived from the internal mammary, and the in- 

 ferior and superior thyroid. The veins end in the internal mammary 

 and left innominate. The nerves come from the sympathetic. 



The vena azygos major is the great link between the venae cavae. 

 Further, it receives the venous blood from the retro-cardiac region, 

 where, of course, no vena cava exists. 



It begins by tributaries from the right lumbar and renal veins, or 

 from the inferior cava itself, and, passing through the aortic opening 

 of the diaphragm, ascends upon the right side of the spine to the 

 fourth dorsal vertebra, whence it turns forward over the root of the 

 right lung to enter the vena cava superior just as it is about to 



