228 On the Structure and Functions of the Organs of Reapiration. 



The structure of the lungs of the chelonian reptiles is very similar. 

 They are very capacious, and have few subdivisions ; and by the quantity 

 of air they contain, materially assist to buoy up the heavy trunk of these 

 animals when sailing on the surface of the water. The chelonia fill their 

 lungs by the deglutition of air, in the same manner as the amphibia, ser- 

 pents, and saurians. 



The respiratory apparatus of birds is intermediate in the perfection of 

 its development between that of the reptiles and of the mammalia. In 

 birds, as in insects, it extends through a great part of the body ; many of 

 the cells of the lungs arc still contained in the abdomen, and are even 

 continued beyond the cavity of the trunk, as under the skin of the neck 

 and extremities. Even the bones are made subservient to this function ; 

 for we find them hollow, and in communication with the lungs. Birds 

 begin their career, however, like reptiles, by having a spongy texture in 

 the interior of their bones, filled with thin marrow. This, in general, is 

 gradually displaced by air j but in the aquatic species, the original condition 

 is retained through life. We may remark, that in birds, as in insects, 

 the great extent of respiratory surface is given by a simple increase in the 

 capacity of the sacs, and not by that minute subdivision into cells, which 

 characterises the highest form of the respiratory organs. No diaphragm 

 exists in birds, excepting in the ostrich, which forms a transition to the 

 class mammalia ; and from the manner in which the lungs are connected 

 with the walls of the chest, the state of fulness is natural, and that of 

 emptiness is forced. The five branchial openings and arteries, which have 

 been already referred to, may be distinctly seen on the neck of the em- 

 bryos of birds, between the third and eighth days of incubation. 



The respiration of quadrupeds is more confined to the lungs, and less 

 extended through the system than in birds ; and though the lungs are pro- 

 portionally less capacious than in birds or in many reptiles, they present an 

 immense surface, owing to their minute subdivision into cells. The mus- 

 cular partition between the two great cavities of the trunk is here com- 

 pleted, and the respiratory organs are confined to the thorax. They are 

 greatly developed in all the more powerful mammalia, as in the carnivorous 

 species ; but they are comparatively smaller in their extent of surface in 

 the feeble and inferiorly-organised herbivora. The varieties of these organs, 

 presented by the different orders of quadrupeds, relate chiefly to their 

 exterior divisions, and- to their greater or less capacity ; the plan of 

 structure being nearly the same in all. 



Thus we observe that there is great uniformity in the development of 

 these organs in the vertebrated tribes ; and a beautiful adaptation in all 

 the forms of the respiratory apparatus, to the living conditions of the ani- 

 mals, and the place they occupy in the scale of beings. 



(To ie concluded in our next.) 



