vi FORM AND FUNCTION g 3 



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 vertebrae by the side of the 

 vertebral artery. On its way to the beak it throws off 

 branches to the vertebra? 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 vertebrae, 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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