300 THE MICROSCOPE AND ITS REVELATIONS. 



same general arrangement prevails; but the cartilaginous reticulation of 

 its upper part projects much further 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 ' kinglets' (Fig. 486, B), each of which has its own bron- 

 chial 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 carti- 

 lage derived from the bronchial tube, A, in the interspaces of which are 

 openings leading to sacculi in their substance. But each of these cavi- 

 ties is surrounded by a solid plexus of blood-vessels, which does not seem 

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

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



FIG. 486. 



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. 



immediate 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 its 

 whole interior is divided-up into minute air-cells, which freely commu- 

 icate with each other, and with the ultimate ramifications of the air-tubes 

 into which the trachea subdivides; and the network of blood-vessels (Fig. 

 487) 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 num- 

 ber of these air-cells grouped around the termination of each air-tube in 

 Man is not less than 18,000; and that the total number in the entire 

 lungs is six hundred millions. 



693. The following list of the parts of the bodies of Vertebrata, of 

 which injected preparations are most interesting as Microscopic objects, 

 may be of service to those who may be inclined to apply themselves to 

 their production. Alimentary Canal; stomach, showing the orifices of 

 the gastric follicles, and the rudimentary villi near the pylorus ; small 

 intestine, showing the villi and the orifices of the follicles of Lieberktthn, 



