CAPILLARIES OF RESPIRATORY ORGANS. 



747 



the interior, that its surface is depressed into sacculi, whose lining 

 is crowded with blood-vessels (Fig. 400). In this manner a set of 

 Air-cells is formed in 



the thickness of the F IG . 399. 



upper wall of the Lung, 

 which communicate with 

 the general cavity, and 

 very much increase the 

 surface over which the 

 Blood comes into rela- 

 tion with the air ; but 

 each Air-cell has a 

 Capillary network of its 

 own, which lies on one 

 side against its wall, 

 so as only to be exposed 

 to the air on its free 

 surface. In the elon- 

 gated Lung of the Snalce 

 the same general ar- 

 rangement prevails ; but 

 the cartilaginous reti- 

 culation 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 communi- 

 cate, one through an- 

 other, with the general 

 cavity. — The structure 

 of the Lungs of Birds 

 presents us with an ar- 

 rangement of a very different kind, the purpose of which is 

 to expose a very large amount of Capillary surface to the in- 

 fluence of the air. The entire mass of each Lung may be con- 

 sidered as subdivided into an immense number of 'lobules 'or 

 ' lunglets' (Fig. 401, b), each of which has its own Bronchial tube 

 (or subdivision of the windpipe), and its own system of blood- 

 vessels, 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 

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

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



Two branchial processes of the Gill of the 

 Ed, showing the branchial lamellae :— a," por- 

 tion of one of these processes enlarged, show- 

 ing the capillary network of the lamelhe. 



