650 



BIRDS. 



has degenerated in the New Zealand parrot (Stringops\ 

 which has ceased to fly and taken to burrowing. 



(d) Air-sacs and air-spaces. The lungs of birds open 

 into a number of air-sacs, which have a larger cubic content 

 than the lungs, and in many cases these air-sacs are con- 

 tinued into the bones, among the viscera, and even under 



the skin. From a 

 broken bone it is 

 possible to inflate the 

 air-sacs, and through 

 a broken bone a bird 



cor. 



with choked wind- 

 g/. . -SC. pipe may for a time 

 breathe. The whole 

 system of air-contain- 

 ing cavities is con- 

 tinuous, except in the 

 case of the skull 

 bones, whose spaces 

 receive air from the 

 nasal and Eustachian 

 tubes. In view of 

 these facts, it used to 

 be supposed that a 

 bird with heated air 

 in the sacs and 

 spaces was compar- 

 able to a balloon. 

 But this is fallacious. 

 i nc }eed 



FIG. 3 2 4 .-Pectoral^irdle and sternum of 



lessen the specific 



A part of carina removed shows peculiar loop of . r i i_ j 



trachea (tr.) ; cl., clavicle; cor., coracoid ; sc., gravity of the bird, 



scapula ; -1., glenoid cavity for head of humerus ; , , r mnnthfllk 



r., parts of sternal ribs. DUt a I6W m( 



of food are sufficient 



to counteract the lightening. Moreover, in many small 

 birds of powerful flight, all the large bones, or all except 

 the humerus, contain marrow, and are therefore not 

 " pneumatic " ; and the hornbill, which has no great power 

 of flight, is one of the most pneumatic of birds. It is 

 certain that in ordinary flight the lightest of birds has to 

 keep itself from falling by constant effort. The bird is 



