Sauropsida: Class Aves 523 



ribs, vertebrae, and even bones of the skull are, in varying degrees, 

 pneumatic. In general, the skeletons of good fliers are more highly 

 pneumatic than those of less capable fliers. Pneumaticity is especially 

 highly developed in large birds which do much sailing or soaring while 

 in flight — e.g., the albatross, frigate bird, and crane. In the flightless 

 ground-living kiwi (Apteryx) of New Zealand, in the wholly aquatic 

 penguin, and in waterfowl such as ducks, there is little pneumaticity 

 except in the skull. Usually the bones of smaller birds are less pneu- 

 matic. There are, however, many exceptional cases — e.g., the hornbills 

 are clumsy fliers but their skeletons are pneumatic to an extraordinary 

 degree. 



All these lesser air-spaces, so far as is known, communicate directly 

 or indirectly (i.e., via adjoining air-spaces) with a major air-sac of the 

 neighboring region of the body. They are all lined by a delicate mem- 

 brane which is continuous with the thin wall of some major air-sac, 

 this being a consequence of the fact that the lesser air-spaces develop 

 by penetration of small ramifying hollow extensions of the primary 

 air-sacs into more or less remote parts of the body. The primary air- 

 sacs are outgrowths from the lungs, and the lungs grow out from the 

 ventral wall of the embryonic pharynx. There is, perhaps, no more 

 extraordinary — indeed, almost incredible — example of specialized 

 modification of the basic general plan of the vertebrate than that all 

 regions of the adult body should be honeycombed with air-filled spaces 

 which are in communication, via the larger air-sacs, lungs, trachea, 

 mouth, and nostrils, with the outside world. Stating the matter in 

 terms of tissues instead of cavities, the endodermal wall of the gill- 

 producing, lung-producing pharyngeal region of the vertebrate ali- 

 mentary tube may give rise to hollow ramifying outgrowths which 

 penetrate the whole body, even the interior of vertebrae and bones of 

 the skull, and sometimes extend almost to the tips of the locomotor 

 limbs. 



This system of air-filled spaces in birds has some structural anal- 

 ogy to the respiratory tracheal system of insects. The epidermis of 

 insects produces numerous delicate tubules which penetrate into all 

 parts of the body, even its deepest organs. But the tracheae are 

 developed by ingrowth from the outermost epithelium (ectoderm) 

 of the animal, whereas the air-spaces of birds are produced by out- 

 growth from the innermost epithelium (endoderm). There cannot 

 be the remotest homology between them, and their physiologic anal- 

 ogy is incomplete because the tracheae are wholly respiratory, but in 

 birds the minute air-spaces remote from the lungs can hardly have any 

 respiratory significance. Both systems, however, are striking examples 

 of extreme departure from the primary structure of a metazoan animal. 



