282 



PULMONARY MUCOUS TISSUE. 



offshoot, which is at first solid, becomes hollow and bifur- 

 cated as it is developed (Fig. 72) : the epithelium at the 

 same time undergoes a change ; from having been pavement 

 in the pharynx it becomes columnar and vibratile in the 

 pedicles of the offshoots (trachea and bronchi), and pave- 

 ment again in the air sacs or pouches (alveoli). The lungs 

 may thus be compared to a gland, the pouches of which are 



Fig. 73. Larynx of a man, trachea, bronchi, and lungs, with the ramification 

 of the bronchi and the division of the lunga into lobules. (Dalton,"Huiuaa 

 Physiology.") 



represented by the alveoli (Fig. 73), and the excretory tubes 

 by the bronchi. These pouches may be likened to a conical 

 and pyriform but indented organ, the summit of which is 

 prolonged by a bronchial ramification : this ampulla (Fig. 

 74), which is about one-eighth of a millimetre in diameter, 

 has not a simple form, but is uniformly embossed on the inside, 

 where it presents a number of prominent folds, dividing the 

 primitive alveolus into a great number of secondary alveoli 



