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, 
Fic. 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 
