Chap. XIIL] 



RESPIRATION. 



155 



The lungs. — The lungs consist of the bronchial tubes and 

 their terminal dilatations, numerous blood-vessels, lympliatics, 

 and nerves, and an abundance of fine, elastic, connective tissue, 

 binding all together. 



The two bronchi, into which the trachea divides, enter the 

 right and left lung respectively, and then break up into a great 

 number of smaller branches which are called the bronchial 

 tubes. The two bronchi resemble the trachea in structure ; but 

 as the bronchial tubes divide and subdivide their walls become 

 thinner, the small plates of cartilage drop off, the fibrous tissue 

 disappears, and the finer tubes are composed of only a thin 

 layer of muscular and elastic tissue lined by mucous membrane. 

 Finally, these finer tubes end in 

 dilated cavities, the walls of \yhich, 

 consisting of a single layer of flat- 

 tened epitheloid cells, surrounded 

 by a fine, elastic, connective tissue, 

 are exceedingly thin and delicate. 



Immediately beneath the layer of 

 flat cells, and lodged in the elastic 

 connective tissue, is a very close 

 network of capillary blood-vessels ; 

 and the air reaching the terminal 

 dilatations by the bronchial tubes is 

 separated from the blood in the cap- 

 illaries by only the thin membranes 

 forming their respective walls. 



The terminal dilatations do not end as simple, rounded sacs, 

 like children's air-balloons, but each bronchiole ends in an 

 enlargement having more or less the shape of a funnel, and 

 called an infundihulum. Each of these infundibula is sub- 

 divided into secondary chambers or cavities, called alveoli^ the 

 walls of which are honey-combed with "bulgings.''^ In this 

 way the amount of surface exposed to the air and covered by 

 the capillaries is immensely increased. ^ 



Fig. 108. — Two Alveoli of 

 THE Lung. Highly magnified. 

 6, 6, bulgiugsof the alveoli, a, a. 



1 These protrusions may be illustrated by a pea-pod, the walls of which are 

 filled with " bulgings," made by the pressure of the peas. 



2 The pulmonary alveoli are often spoken of under the general name of air- 

 sacs, and the " bulgings " are known as air-cells. The term "air-cells," though 

 common, is misleading. 



