VI FORM AND FUNCTION 83 
the inner coat and act as buttresses. Sometimes these 
buttresses are bound together and a strong network is 
formed. It is a network like this which supports the 
beak. In the skull the plates take the form of arches. 
In all birds without exception, I believe, some of the 
bones of the skull are aerated, the air being derived 
mainly from the nostrils and ears. But the beak and 
some of the bones connected with it are aerated from 
the lungs. Thither runs from each cervical air-sack a 
small tube of membrane which lies in an incomplete 
bony canal under the vertebre by the side of the 
vertebral artery. On its way to the beak it throws off 
branches to the vertebrz of the neck. Every aerated 
bone has a foramen or aperture through which the bag 
of membrane finds its way. In the humerus it is easy 
to find at the end near the body on what is properly the 
upper side of the bone, but which in the bird’s wing, 
when it is folded, looks backward. The interclavicular 
sack opens into it. Of all the long bones the humerus 
is most commonly pneumatic. An easy and interest- 
ing experiment is to tie up the windpipe of a dead bird, 
then break the humerus and blow down it through a 
blowing tube, when the sacks will at once inflate. 
Indeed wounded birds when their windpipes have been 
choked with blood have been known to breathe through 
a broken humerus that has pierced the skin. Other 
bones that are frequently aerated are the breast- 
bones, the coracoid, the vertebra, less frequently the 
thighbone, shoulder-blade and merrythought. But a 
good many birds are, as I have said, pneumatic to the 
very extremities, the Hornbills and the Screamers to 
the ends of the fingers and toes. The Gannet has 
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