82 



THE VITAL PROCESSES 



The Alveoli. The alveoli, or air cells, are the small divi- 

 sions of the inf undibula (Fig. 36). They are each about one 

 one-hundredth of an inch (| mm.) in 

 diameter, being formed by the infolding 

 of the infundibular wall. This wall, 

 which has for its framework a thin 

 layer of elastic connective tissue, sup- 

 ports a dense network of capillaries 

 (Fig. 37), and is lined by a single layer 

 of cells placed edge to edge. By this 



arrangement the air within the alveoli 

 FIG. 36. Termi- 

 nal air sacs. The 1S brought very near a large surface 



two large sacs are in- of blood, and the exchange of gases 

 between the air and the blood is made 

 possible. It is at the alveoli that the 

 oxygen passes from the air into the 



blood, and the carbon dioxide passes from the blood into 



the air. At no place in 



the lungs, however, do 



the air and the blood 



come in direct contact. 



Their exchanges must 



in all cases take place 



through the capillary 



walls and the layer of 



cells lining the alveoli. 

 Blood Supply to the 



Lungs. To accomplish 



the purposes of respira- 

 tion, not only the air, 



fundibula; the small 

 divisions are alveoli. 

 (Enlarged.) 



FIG. 37. Inner lung surface (magni- 



fied), the blood vessels injected with color- 

 but the blood also, must ~, . .. 



mg matter. I he small pits are alveoli, 



be passed into and from and the vessels in their walls are chiefly 

 the lungs. The chief capillaries. 



