IV. 



The Air-sacs and Hollow Bones of Birds. 



NOT very long ago the problems connected with this subject were 

 settled in a very offhand way. The heated air in the sacs being 

 lighter than the surrounding air, made the bird a balloon, and so flight 

 was easy. For the same object, the bones were hollow and marrow- 

 less. Thus a clear and interesting solution seemed to have been 

 found for a great problem. 



This theory has withered beneath the cruel light of fact. A 

 bird can carry only a very small amount of air in its sacs, and the 

 difference in weight between a few cubic inches of heated or cold air 

 is too infinitesimal to be worth considering. The sight of an eagle 

 flying off with a lamb ought to convince anyone who cannot otherwise 

 be convinced, that the saving of the tiniest fraction of an ounce of 

 weight would make no difference. True, air within the bird, whether 

 heated or not, will expand its volume, lessen its specific gravity, and 

 so fit it better for floating, but it could not help it to rise, and this is 

 the real difficulty. Moreover, many birds, for instance the swallow, 

 which fly to perfection, have all their large bones solid. 



We must, therefore, look for some sounder theory, and first it will 

 be well briefly to survey the facts. The lungs of birds open out into 

 great membranous expansions which lie within the body-cavity next to 

 the ribs or extend far back between the kidneys and the intestines ; and, 

 in addition to this, in many cases, the membrane finds its way into 

 the bones as the marrow dries up, sometimes even to the very 

 extremities of the limbs, and in some instances under the skin also, 

 into some of the feathers and between the muscles. Anyone who 

 doubts the connection between the chambers within the bones and the 

 lungs, may convince himself by a very simple experiment. He can 

 take a dead bird (any bird which has some of its bones aerated), 

 break the humerus, and, after tying up the trachea, blow down the 

 bone through a tube, when all the air-sacs will expand as promptly 

 and completely as if the tube were inserted into the trachea. 



Before now, a wounded bird, whose windpipe has been stopped 

 with blood, has been known to breathe through a broken and exposed 

 bone. It is beyond a doubt, then, that the hollow bones are lined 

 with expansions of the bronchial membrane ; it extends, in fact, even 

 to some parts of the skull. 



I shall first discuss the functions of the air-sacs proper as 

 distinguished from their extensions into the bones. It cannot be 



