THE THORAX 951 



notch. (3) The right lung is larger and heavier than the left, the 

 weight of the right being about 20 ounces and that of the left 

 about 18 ounces. (4) The rigit lung is shorter than the left, this 

 being due to the fact that the liver causes the right half of the dia- 

 phragm to rise higher than the left half. (5; The right lung is 

 broader than the left, because the heart projects more to the left 

 side than to the right. 



Vertical Extent of the Lungs. — In the mammarj' line the right 

 lung descends as low as the sixth rib ; in the mid-axillary line, as 

 low as the eighth rib ; and, in the scapular line (inferior angle of 

 the scapula), as low as the tenth rib. The lower limits of the left 

 lung exceed those of the right by about the depth of a rib. 



Root of the Lung. — ^The root is situated at the hilum pulmonis 

 on the inner surface. Its chief constituents are as follows : (i) the 

 bronchus or air-tube ; (2) the pulmonary artery, which conveys 

 venous blood to the lung ; and (3) the two pulmonary veins, which 

 convey the arterial or oxygenated blood from the lung to the left 

 auricle of the heart. In addition to these constituents there are 

 (a) the bronchial arteries and veins, (6) the pulmonary' lymphatic 

 vessels, (c) the pulmonary nerves, and (d) the bronchial Ijinphatic 

 glands. All these constituents are connected by areolar tissue, and 

 the entire root is invested by the pleura. 



Relations. — The following relations are common to both roots : 

 in front, the phrenic nerv^e, with the superior phrenic artery, 

 and the anterior pulmonary plexus of nenes ; behind, the pneumo- 

 gastric nerve and the posterior pulmonary plexus of nerves ; and, 

 below, the ligamentum latum pulmonis. 



Special Relations — Right Root — Anterior. — The superior vena 

 cava and the upper part of the right auricle of the heart. 

 Superior. — ^The right azygos vein, as it arches forwards over the 

 right bronchus to open into the superior vena cava. Posterior. — 

 The right azygos vein. Left Root — Superior.— The arch of the 

 aorta. Posterior. — ^The descending thoracic aorta. 



Relative Position of the Chief Constituents.— The relation from 

 before backwards is the same on each side, and is as foUows : (i) the 

 upper of the two pulmonary veins ; (2) the pulmonary artery ; 

 and (3) the bronchus. The relation from above downwards differs 

 on the two sides. On the right side the relation is bronchus, pul- 

 monary artery, and pulmonary veins ; and, on the left side, pul- 

 monary artery, bronchus, and pulmonary veins. 



The difference in the position of the bronchus on each side is 

 due to the fact that the right bronchus gives off a branch known 

 as the eparterial bronchus, which lies above the right pulmonary 

 artery, the larger portion of the right bronchus and the whole of 

 the left bronchus being hyparterial. 



Straetare of the Langs — The trachea divides into two bronchi, right 

 and left, the structure of which is similar to that of the trachea. The right 

 bronchos, at about J inch from its origin, gives oflF superiorly a branch 



