VOCAL AND RESPIRATORY APPARATUS. 253 



accurately fill the cavity of the chest at all times,, and are covered 

 by the pleura. They are conical in shape, presenting each a base, 

 apex, two borders, and two surfaces. The apex extends upward 

 above the level of the first rib; the base occupies the convex 

 surface of the diaphragm; the external, or thoracic surface is 

 accurately applied to the wall of the thorax ; the inner surface is 

 in contact with the pericardium, and is marked by a depression, 

 the hilum pulmonis, at the root of the lungs. The posterior bor- 

 der rests on either side of the spinal column, and the anterior 

 border is thin and overlaps the pericardium. 



The root'of each lung, situated near its middle, is composed 

 of the following structures, surrounded by a reflection of 

 pleura : 



Bronchus; 



Pulmonary artery; 



Pulmonary veins; 



Bronchial glands; 



Bronchial vessels; 



Posterior and anterior pulmonary plexuses of nerves; 



Connective tissue. 



The root of the right lung is behind the right superior 

 cava and the vena azygos arches over it. The root of the left 

 lung lies below and in front of the arch and descending aorta. 

 The relative positions of the pulmonary veins, pulmonary artery, 

 and bronchus on either side are: 



Both sides, from before backward 



V. Pulmonary veins, 



A. Pulmonary artery, 



B. Bronchus. 



Eight side, from above downward 



B. Bronchus, 



A. Pulmonary artery, 



V. Pulmonary veins. 



Left side, from above downward 



A. Pulmonary artery, 



B. Bronchus, 



V. Pulmonary veins. 



The weight of the lungs varies according to many conditions. 

 In the adult the approximate weight is about forty-two ounces, 

 the right being two ounces heavier than the left, and their total 

 capacity about three hundred cubic inches. The right lung has 

 three lobes, the left but two, of which the lower is the larger. 



