364 RESPIRATION. 



bronchiole becomes enlarged, forming the atrium, which opens 

 into sac-like cavities, infundibula, each infundibulum being about 

 1 mm. in diameter, and these open into air-cells or pulmonary 

 alveoli, which latter have a diameter of from 0.1 mm. to 0.3 mm. 



At the infundibula the muscular tissue is less abundant and 

 the elastic fibers are arranged around the openings of the air-cells. 

 The epithelium in the bronchioles is both columnar ciliated and 

 cubical non-ciliated ; but in the infundibula and alveoli it is of the 

 pavement variety, with some cubical. 



Blood-vessels. Branches of the pulmonary artery pass into 

 the lung with the bronchial tubes and terminate in the pulmonary 

 capillaries (Fig. 205), which as plexuses lie under the mucous mem- 

 brane of the walls of the infundibula and alveoli and of the par- 

 titions or septa between them. The capillaries have very thin 



Blood-capillaries 

 seen in surface 



"" Alveolus in cross- 

 section. 



FIG. 205. Section through injected lung of rabbit (Bohm and Davidoff). 



walls and a diameter of about 8 p, while the spaces between them 

 are even smaller. Small veins collect the blood, and these uni- 

 ting with others finally discharge into the pulmonary veins, which 

 bring the blood back to the left auricle. Some of the blood brought 

 to the lungs by the bronchial arteries returns to the venous circula- 

 tion through these veins. 



The bronchial arteries, branches of the aorta, supply the blood 

 necessary to nourish the lung tissue, and the bronchial veins carry 

 most of it back to the venous circulation, by the way of the vena 

 azygos major on the right side, and by the superior intercostal or 

 left azygos vein on the left side. 



Nerves. The innervation of the lungs is supplied through 

 the pulmonary plexuses from the sympathetic and vagus. The 

 nerves accompany the bronchial tubes. 



