LUNGS OF BIKDS AND MAMMALS. 



625 



one side against its wall, so as only to be exposed to the air on 

 its free surface. In the elongated lung of the Snake, the same gene- 

 ral arrangement prevails; but the cartilaginous reticulation of its 

 upper part projects much further into the cavity, and encloses 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 may be considered 

 as subdivided into an immense number of " lobules" or "lung- 

 lets" (Fig. 332), each of which has its own bronchial tube (or 



FIG. 332. 



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. 



subdivision of the windpipe), and its own system of bloodvessels, 

 which have very little communication 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, in the in- 

 terstices of which are openings leading to sacculi in their sub- 

 stance. But each of these cavities is surrounded by a solid plexus 

 of bloodvessels, which does not seem to be covered by any limit- 

 ing membrane, but which admits air from the central cavity 

 freely between its meshes ; and thus its capillaries are in imme- 

 diate relation with air on all sides, a provision that is obviously 

 very favorable to the complete and rapid aeration of the blood 

 they contain. 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 the 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 (windpipe) subdivides ; and the network of bloodvessels 

 (Fig. 333) 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 



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