VI FORM AND FUNCTION 107 
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 
vertebrz, the ribs, the breast bone, and the coracoids. 
(§) 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. 
