1064 



VERTEBRATED ANIMALS 



general arrangement prevails ; but the cartilaginous reticulation 

 of its upper part projects much farther into the cavity, and incloses 

 in its meshes (which are usually square, or nearly so) several layers 

 of air-cells, which communicate, one through another, with the 

 general cavity. The structure of the lungs of birds presents us with 

 an arrangement of a very different kind, the purpose of which is to 

 expose a very large amount of capillary surface to the influence of the 

 air. The entire mass of each lung may be considered as subdivided 

 into an immense number of ' lobules ' or 'lunglets' (fig. 801, B), each of 



FIG. 801. Interior structure of lung of fowl, as displayed by a section, A 

 passing in the direction of a bronchial tube, and by another section B 

 cutting it across. 



FIG. 802. Arrangement of the capillaries 011 the walls of the air-cells of 

 the human lung. 



which has its own bronchial tube (or subdivision of the windpipe) 

 and its own system of blood-vessels, which have very little com- 

 munication with those of other lobules. Each lobule has a central 

 cavity, which closely resembles that of a frog's lung in miniature, 

 having its walls strengthened by a network of cartilage derived from 

 the bronchial tube, A, in the interspaces of which are openings lead- 

 ing to sacculi in their substance. But each of these cavities is sur- 

 rounded by a solid plexus of blood-vessels, which does not seem to be 

 covered by any limiting membrane, but which admits air from the 



