360 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



out with epithelial membrane — an extension of the mu- 

 cous membrane of the mouth and throat — webbed to- 

 gether with loose connective tissue and invested with 

 serous membrane, and we shall have a sufficiently clear 

 idea of the general structure of the lungs. 



The obvious purpose of all this is to expose as much 

 surface as possible to the oxygen of the air. The 

 minuter the ramification, the smaller the terminal cells; 

 or the finer the sponge, the 

 larger will be the surface. 

 The area of the epithelial 



Fig. 241. — Termination of a capil- 

 lary bronchus : a, bronchus ; 6, 

 cell : c, cellule. 



Fig. 242. — Showing the capillary 

 blood vessels (b) ramifying on 

 the surface of the cells (a). 



surface exposed in the lungs has been variously esti- 

 mated from four hundred to fifteen hundred square feet. 

 Now beneath this extensive epithelial surface the blood 

 vessels ramify most minutely and exchange with the air 

 C0 2 for oxygen (Fig. 242). 



But to make this exchange effective both the air and 

 the blood must be in constant circulation. When the 

 blood has discharged its C0 2 and taken in its supply of 

 O it must get out of the way for other blood to do the 

 same. Similarly, when the air has given up its O and 

 taken in its supply of C0 2 , it must move on and give 



