72 



ZOOLOGY. 



it.* Finally, inferiorly, the trachea subdivides into two 



branches called bronchi, which ramify in both lungs like 



the roots of a tree in the soil (c e, Fig. 49). 



136. The lungs show 

 internally a vast number of 

 small cells, into each of which 

 a branch of the correspond- 

 ing bronchus opens. A soft, 

 delicate, and vascular mem- 

 brane lines the walls of these 

 cells and air tubes, and it 

 is to these that the alternate 

 branches of the pulmonary 

 artery are distributed, by 

 means of which, now become 

 capillaries, the venous blood 

 is exposed to the action of 

 d the air. 



The smaller the cells, the 

 greater will be the extent of 

 the membrane, and the more 

 extended the surface upon 

 which the blood is exposed 

 to the action of the air. 

 The smallness of the cells 

 and the activity of respira- 

 tion are thus in a direct 

 ratio to each other; and 



this is proved by contrasting the large pulmonary cells in 



the lungs of the frog with the microscopic cells which we 



find in the lungs of birds and mammals. 



137. But air, as air, never penetrates beyond the little 



cells or cul de sacs, in which the air tubes terminate in mam- 



* The surface of the membrane of the trachea is covered with a fine down, 

 each hair of which exhibits vibratile movements, called also ciliary. This 

 vibratile movement determines in the liquid in contact with their surface, 

 currents, which are often very rapid, and which persist even after a portion 

 of the membrane has been removed Irom the Tbody. The cilia are micro- 

 scopic. The direction of the current seems to be from the exterior towards 

 the interior, and the same movements may be observed on the surface of 

 the nasal fossae, but nothing of the kind is to be seen in the pharynx. 



t One of the lungs is left untouched (d) ; but in the other the substance has 

 been destroyed, in order to expose the right bronchus and its ramifications in 

 the lung. 



a, larynx and superior extremity of the trachea ; b, trachea ; c, division 

 into bronchi ; </, one of the lungs : e, bronchic ramuscules. 





Fig. 49. Lungs and Trachea 

 in Man. t 



