LUNGS 1065 



central cavity freely between its meshes ; and thus its capillaries are 

 in immediate relation with air on all sides a provision that is ob- 

 viously very favourable to the complete and rapid aeration of the blood 

 they contain. 1 In the lung of man and mammals, again, the plan of 

 structure differs from the foregoing, though the general effect of it is 

 the same. For its whole interior is divided up into minute air-cells, 

 which freely communicate with each other, and with the ultimate 

 ramifications of the air-tubes into which the trachea subdivides ; and 

 the network of blood-vessels (fig. 802) is so disposed in the partitions 

 between these cavities that the blood is exposed to the air on both 

 sides. It has been calculated that the number of these air-cells 

 grouped around the termination of each air-tube in man is not less 

 than eighteen thousand, and that the total number in the entire 

 lung is six hundred million*. 



1 On the respiratory organs of birds, see Campana, La Respiration tli's Oiseai/x, 

 Paris, 1875. 



