112 TOPOGRAPHIC AND APPLIED ANATOMY. 



PLATE 10. 



The vessels and nerves of the mediastinum and their relation to the cervical viscera. The first illustration repre- 

 sents the structures as seen from the left, while the second shows them as seen from the right. (Formalin preparation.) 



ment. In this manner the cardiac dulness may be displaced to the left beyond the mammary 

 line, by a contraction of the left lung and of its pleural cavity. 



The Thymus Gland. Immediately behind the sternum, enclosed between the two layers 

 of the mediastinal pleura and extending upward toward the neck, are found the remains of the 

 thymus gland, which usually have undergone fatty degeneration, but which still retain their 

 original bilobed structure. During childhood the grayish-red gland may be seen glistening 

 beneath the pleura. If the layers of the mediastinal pleura are dissected back, we penetrate 

 further into the mediastinal space and readily expose the superior vena cava with the superim- 

 posed right phrenic nerve (see Plate 10). The superior vena cava produces a deep groove upon 

 the median surface of the right pulmonary apex (see Fig. 47). It arises at the level of the sternal 

 insertion of the first rib upon the right by the junction of the longer left with the shorter right 

 innominate vein, and is easily injured by a penetrating wound close to the right sternal margin. 

 Each innominate vein is formed behind the corresponding sterno-clavicular articulation by the 

 union of the internal jugular with the subclavian vein. The left innominate vein does not pro- 

 ject above the superior margin of the sternum (see page 68) ; it receives the middle thyroid veins. 

 The vena azygos major (see page 116, Figs. 49 and 51, and Plates 7 and 8) passes from the ver- 

 tebral column over the right bronchus to empty into the superior vena cava posteriorly just 

 before this vessel enters the pericardium. 



The Ascending Aorta. The ascending aorta is situated within the pericardium to the left 

 of the superior vena cava; still further to the left is the pulmonary artery. To avoid confusing 

 the aorta with the pulmonary artery, a common mistake of the tyro, it should be clearly noted 

 that the commencement of the aorta crosses behind the pulmonary artery, so that the aorta, 

 although arising from the left ventricle, is situated to the right of the pulmonary artery and is the 

 middle vessel of the three great trunks at the base of the heart. Aneurysms of the ascending 

 aorta frequently rupture into the pericardial cavity, producing a fatal hemorrhage. This vessel 

 is about six centimeters in length; it runs backward and downward toward the left and then 

 forward, upward, and to the right [by English authors the ascending aorta is described as running 

 forward, upward, and to the right ED.], so that it gradually approaches the inner surface of 

 the thoracic wall. The origin of the aorta is at the level of the upper border of the sternal attach- 

 ment of the third left costal cartilage, six centimeters behind the sternum; at the level of the 

 first intercostal space it is, however, only two centimeters behind the sternum [i. e., behind the 

 right margin of the sternum ED.]. The right convex border of the aorta extends to the right 

 sternal margin. It is in this situation that aneurysms of the ascending aorta are particularly 

 prone to press forward the anterior thoracic wall and rupture externally; they may also occasion- 

 ally perforate into the right pleural cavity. Such an aneurysm may press upon the superior 

 vena cava on the right and cause symptoms of venous stasis in the region drained by this great 

 trunk ; it may press upon the pulmonary artery on the left and lead to a hypertrophy of the right 

 ventricle (Plate 9). Rupture into both of these vessels has also been observed. The most im- 



