THE RESPIRATORY SYSTEM OF MAMMALS 



155 



saccular diverticula, either alveolar sacs or siii(i;le i)uhnouary alveoli. 

 (Fi^. 9S.) The fil)rous tissue, elastic fil)ers, and smooth muscle 

 cells are chiefly in the wall of the aheolar duct, around the openings 

 to the alveolar sacs and pulmonary alveoli. The alveolar sacs are 

 not single bladder-like enlargements, but each consists of a cluster 

 of respiratory or pulmonary alveoli which are homologous with the 

 secretory end-pieces of a gland like the parotid. Alveolar sacs and 

 respiratory alveoli are mere sacs whose walls appear to consist of 

 squamous epithelium and re- 

 ticular tissue. The alveoli 

 often contain large cells called 

 dust cells, since they contain 

 black particles. Their source 

 is debatable. 



Each pulmonary unit, there- 

 fore, consists of a respiratory 

 bronchiole and its alveolar sacs 

 and pulmonary aheoli. The 

 air sacs and alveoli are in- 

 vested with a fine, close- 

 meshed network of large capil- 

 laries, so thick-set that the 

 spaces between adjacent capil- 

 laries is less than the width 

 of the capillaries. Between 

 adjacent alveoli there is also 

 a rich meshwork of reticular 

 fibers, some elastic fibers, and 

 scattered smooth muscle cells. 



Vascular Supply. ^The lungs are supplied by two sets of blood 

 vessels, namely, («) the bronchial arteries and veins, and (b) pul- 

 monary arteries and ^•eins. The l^ronchial \'essels arise from the 

 systemic system and their branches extend in the connecti\e tissue 

 down into the interlobular connective tissues. The pulmonary 

 artery arises from the right ventricle and has a branch to each limg, 

 w^here its branches follow those of the bronchi. Alongside a ter- 

 minal bronchiole is a very small branch of a pulmonary artery. 

 This branches as the respiratory bronchioles are formed, so that near 

 the alveolar ducts and sacs the pulmonary arterioles break into 

 capillaries which invest the air cells. At the lateral borders of the 

 alveoli of a pulmonary unit, the small venules originate which 



Fig. 9s. — Photograph of the king of a 

 rabbit, showing a small bronchiole in 

 upper right. In Ihe center a respiratory 

 bronchiole is shown breaking into two 

 alveolar ducts and their alveoli. 



