ORGANS OF RESPIRATION'. 561 



their interior is more uniformly cellular throughout than in 

 reptiles, and the cellules are larger than in mammalia, giving 

 a light spongy texture to these organs, which occupy a com- 

 paratively small space in the alvi-thorax of birds. Meckel 

 found the lungs of an emaciated eagle to weigh only -^ of the 

 entire carcase, and those of an equally spare heron -i-H- j they 

 are proportionally largest in the singing birds, and least in 

 the strutheous and other heavy species. 



The trachea, like the neck, is necessarily lengthened in this 

 class, and the tracheal rings, like the cartilages of other parts 

 of their body, are generally ossified, and thus preserve a free 

 passage during the various contortions and pressures to which 

 this part is exposed. Surrounded with entire ossified rings 

 throughout its course, it descends along the left side of the 

 oesophagus and crop, into the cavity of the trunk ; commencing 

 above in a larynx, simple and uniform in structure, and much 

 resembling that of chelonian reptiles, it terminates below 

 the clavicles in a complicated osseous moveable apparatus, the 

 inferior larynx, for modulating the vocal sounds ; here the 

 trachea bifurcates to form the two soft and partially annulated 

 bronchi, which, without any previous subdivision, penetrate 

 the ventral surface of the lungs, a little behind their thick 

 anterior margin. The trachea is often lengthened by convo- 

 lutions at its lower part in aquatic birds, as swans, cranes, 

 geese, pintados, demoiselles, and spoonbills ; the larynx and 

 tracheal rings, are cartilaginous in the strutheous and some 

 other birds, as in mammalia. The canal of the trachea, 

 though commonly cylindrical, is often varied, especially in 

 the male, by constrictions and dilatations in its course, as 

 in mergus, anas, and other palmipedes, where the rings con- 

 tinue complete around the dilated parts. The tracheal 

 rings are sometimes incomplete and open at the dilated part, 

 as first shown by Fremery, in both sexes of the emeu of 

 New Holland, where they leave a free membranous tracheal 

 pouch a little above the sternum. The rings are often sutured 

 on one side ; they are remarkably destitute of bilateral sym- 

 metry in cranes and some other wading birds ; and the lower 

 part of the trachea is sometimes divided internally by a longi- 

 tudinal membranous septum, as in procellaria and aptenodytes. 

 For the retraction of the long trachea of birds, it is commonly 

 provided with a superficial and a deep-seated pair of retractor 



PART VI. O O 



