THE RESPIRATORY SYSTEM. 161 
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 AS 
a4 
reflected at the root or artery 
the lung, where the blood- = 
vessels and bronchus enter, air ¢ube 
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- 
: Seis Fic. 84. DracrAM SHOWING HOW THE 
rior or ventral mediasti- Arr Tuse or BroncHIoLte TERMI- 
num contains blood vessels a eae 
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 alveoh, 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. 
in, infundibulum with air sacs over 
which numerous capillaries lie. 
15 
