vi FORM AND FUNCTION 



1 07 



capillaries in the great expanse of bronchial membrane 

 must help a little to aerate the blood. With a view to 

 discovering the main purpose of pneumaticity, I will 

 briefly set down the chief facts. 



(1) Many small birds that are first-rate flyers have 

 either marrow in all their larger bones, or else in 

 all except the upper-arm bone ; the Swift in all with 

 this exception, the Swallow in all. 



(2) Most of the big strong-flying birds have a great 

 deal of aeration. 



(3) The Hornbills, which according to good ob- 

 servers are very poor flyers, are as pneumatic as any 

 birds or, perhaps, more so than any. 



(4) The Apteryx, the wingless bird of New Zealand, 

 has only part of its skull, and no other bones, aerated. 

 On the other hand the Ostrich, Emeu, Rhea, and 

 Cassowary have great hollows in the thigh bones, the 

 vertebrae, the ribs, the breast bone, and the coracoids. 



(5) Birds which dive have solid bones, or only the 

 shoulder bone aerated. 



(6) Birds which spend much of their time in the 

 water without diving have, at least in all the species 

 of which I have been able to obtain specimens, nearly 

 all the bones solid. The Gulls are the most striking 

 example of this, even the humerus in. the Black- 

 headed Gull being solid. 



(7) There are great differences between nearly 

 related species — e.g. the Gannet has an extraordinary 

 amount of aeration, while its near ally, the Cormorant, 

 has only the humerus pneumatic. The Hornbill is 

 not very distantly related to the Swift, which has 

 singularly little aeration. 



