THE RESPIRATORY SYSTEM. 



161 



vein 



may be expanded by tying a piece of glass tubing on the 

 trachea and blowing into it strongly for a few seconds^ 

 Each lung is completely 

 invested by a sac of deli- 

 cate transparent serous 

 membrane called pleura 

 (Fig. 82). Each sac is 

 reflected at the root or 

 the lung, where the blood- 

 vessels and bronchus enter, 

 so as to form a parietal 

 layer lining its half of the 

 thoracic cavity. The me- 

 dian space between the 

 two sacs is called the 

 mediastinum. The ante- 

 rior or ventral mediasti- 

 num contains blood vessels 

 and the thymus gland. 

 The dorsal or posterior 

 mediastinum lodges the trachea, esophagus and aorta and 

 the heart occupies the middle mediastinum. 



Each lung is divided by deep clefts into several lobes. 

 The left lung is composed of two large lobes and a small 

 one. The right lung consists of four unequal lobes. The 

 cranial end of the lung is the apex and the caudal end, rest- 

 ing against the diaphragm, is the base. The bronchi, as 

 they are continued into the lungs, subdivide into smaller 

 tubes, whose later subdivisions are the bronchioles. The 

 latter, dividing like the branches of a tree, finally terminate 

 in blind pouches known as infundibula or alveoli, the walls 

 of which are thickly beset with microscopic sac-like evagina- 

 tions named air sacs (Figs. 83 and 84). The walls of 

 these air sacs are very thin, somewhat like the peritoneum. 

 15 



FIG. 84. DIAGRAM SHOWING HOW THE 

 AIR TUBE OR BRONCHIOLE TERMI- 

 NATES IN INFUNDIBULA. 



in, infundibulum with air sacs over 

 which numerous capillaries lie. 



