6i6 



BIRDS. 



windpipe may for a time breathe. The whole system of 

 air-containing cavities is continuous, 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 comparable to a balloon. But this is fallacious. The 

 air must indeed lessen the specific gravity of the bird, but 

 a few mouthfuls 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 there- 

 forenot "pneumatic"; 

 and thehornbill, 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 not 

 comparable to a bal- 

 loon, but to a flying 

 machine ; "it has to 

 be not a buoyant cork, 

 but a buoyant bullet." 

 In short, the air-sacs 

 increase the bird's respiratory content, secure more perfect 

 aeration of the lungs, and probably aid in regulating the 

 body temperature. 



Ruskin has compared the flight of a bird to the sailing of a boat. 

 " In a boat the air strikes the sail ; in a bird the sail strikes the air ; in 

 a boat the force is lateral, and in a bird downwards ; and it has its sail 

 on both sides." But, as he says, the sail of a boat serves only to carry it 

 onwards, while wings have not only to waft the bird onwards, but to 

 keep it up. To carry the weight of the bird the wings strike vertically, 

 to carry the bird onwards they strike obliquely ; sometimes the direc- 

 tion of the stroke is more vertical, and then the bird mounts upwards ; 

 sometimes it is more oblique, and then the bird speeds onwards ; usually 



Fig. 271. — Position of wings in pigeon at 

 maximum elevation. —From Marey. 



