FLIGHT. 



675 



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 with choked 

 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. The air must 

 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 therefore not "pneumatic" ; and the hornbill, 



FIG. 375. Position of wings in pigeon at maximum elevation. 

 From Marey. 



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 air-sacs increase 

 the bird's respiratory content, secure more perfect aeration of the 

 lungs, and assist in internal perspiration, thus helping in the regulation 

 of the body temperature. 



To carry the weight of the bird, the wings strike vertically ; to carry 

 the bird onwards, they strike obliquely. Sometimes the direction of the 

 stroke is more vertical, and then the bird mounts upward ; sometimes 

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



