THE LUNGS OF BIRDS. 863 



it in, as is the case in Mammalia. The lungs of Birds are somewhat flattened, 

 and fixed to the back of the thorax ; they are relatively smaller than in Mam- 

 malia ; their lobules are very distinct, each having its own bronchial tube and 

 bloodvessels. Their interior is extremely subdivided, or cellular ; the sacculi 

 or cells thus formed are at first supported by delicate cartilaginous trabeculse ; 

 but some open into the ultimate air-cells. These cells are small ; whilst the 

 capillaries upon them are exceedingly numerous, and their network very close; 

 and owing to the frequent communications between neighboring clusters of 

 air-cells, and to other minute arrangements, come into relation with the air 

 on both sides. These capillaries when injected seem to be varicose, and even 

 to project into the air-cells, in such a manner as to appear naked, or not 

 covered by mucous membrane ; this view is adopted by some, though, more 

 probably, an exceedingly delicate membrane exists upon them. 



The high temperature, the active habits, and the rapid waste of tissue in 

 Birds, are associated with a corresponding activity of the respiratory function. 

 These animals absorb a larger amount of oxygen, and exhale more carbonic 

 acid, in relation to their weight, than the Mammalia ; they are also much 

 more dependent on a due supply of pure air, than the latter, and are much 

 more quickly asphyxiated. Two supplementary anatomical conditions, pecu- 

 liar to Birds, must more or less aid in their active respiration. First, there 

 are usually found in the neck, thorax, and abdomen, and even in the limbs, 

 membranous bags, named air-sacs, into which the air gains access by exten- 

 sions from certain bronchial tubes, which reach to the surface of the lungs, 

 and there communicate with the thoracic or pleural air-sacs, from which other 

 communications extend to the abdominal and remaining air-cavities. These 

 air-sacs are highly elastic, have a few plain muscular fibres in their walls, and 

 are lined by a fine, moderately vascular, and partially ciliated mucous mem- 

 brane. Secondly, in many Birds most of the bones are hollow, and are filled 

 with air. In certain Mammalia some of the bones of the face and cranium 

 contain air ; but, in most Birds, besides these bones, the vertebrae and sternum, 

 and even the long bones, which in Mammalia and in the early condition of the 

 Bird, contain marrow, such as the clavicle, humerus, and femur, and even the 

 merrythought and shoulder bones, are, in the full-grown Bird, occupied with 

 air, which finds access to their interior by special membranous canals leading 

 from the adjacent air-sacs. When the trachea is tied respiration may be per- 

 formed for a time through an aperture made in the arm-bone. These cavities 

 in the bones are lined with a membrane which, as compared with that of the 

 air-sacs, is highly vascular. In Birds killed suddenly, or destroyed slowly by 

 drowning, the air in the air-sacs and bones is often charged with from 8 to 

 even 15 per cent, of carbonic acid (Dr. Davy) ; respiratory interchanges of 

 oxygen and carbonic acid probably, therefore, here take place between the air 

 and the blood, and these cavities must be regarded, not so much as supple- 

 mentary, as sub-respiratory chambers, for the increase of the surface of absorp- 

 tion and exhalation. But their importance, in this respect, has perhaps been 

 exaggerated. An equal extension of respiratory surface, if that only were 

 needed in the economy of the Bird, might have been obtained by a trifling en- 

 largement of the lungs themselves. The membrane lining the cavities of the 

 bones is not so vascular as a respiratory membrane usually is, whilst that of 

 the capacious air-sacs is still less so ; moreover, there are some Birds which 

 have no air in the long bones, or even in other bones ; such exceptions occur 

 in various Orders, chiefly, however, amongst the smaller Birds, and some 

 aquatic species ; lastly, the Apteryx has no air in any of its bones, and is even 

 destitute of air-sacs, excepting the pleural chambers, in this respect being 

 quite singular. 



The high temperature of Birds has probably some other explanation than 

 the presence of these sub-respiratory air-chambers. As already mentioned 

 (p. 191), these air-sacs, and the air-cavities in the bones, cannot much dimin- 

 ish the weight of a Bird in the air, by the relative temperature of their con- 

 tents, but they may aid in flight, by their distending and gjving fixity to the 

 thorax, which is the base of action for the wings ; they may also, by the dif- 

 ferent pressure which is exercised upon them during flight,' act as a sort of 



