264 VERTEBRATE ZOOLOGY 



the intestines, in such a condition that digestion, or the refining of the 

 fuel, is rapid and complete. Much might be said of the efficiency of 

 the excretory apparatus, but this may be assumed. 



The mechanics of propulsion is difficult of explanation because 

 of its extreme complexity; but this much may be said: the wing stroke 

 is practically like the arm stroke in swimming. It must do two things: 

 prevent the body from falling, and give a forward impulse. The stroke 

 must therefore be downward and backward; but a forward and up- 

 ward stroke, like the recovery stroke in swimming, alternates with the 

 power stroke. The possibility of effective and rapid propulsion de- 

 pends on the relatively frictionless character of the recovery stroke. 

 This is accomplished by bringing back the wing edgewise to the re- 

 sistance of the air. Many birds make progress by planing up and down 

 the air currents with nearly rigid wings. In this phase of flight man 

 has equaled, if not surpassed, the bird. 



3. Lightness and Rigidity. — Many elements combine to make the 

 bird a model of mechanical perfection in this respect. The skeleton 

 (Fig. 144) exhibits instances of the use of nearly all of the recognized 

 architectural principles designed for getting the most strength and 

 rigidity out of the least material. The T and I beam principles are 

 used in many of the bones, the most striking example being the ster- 

 num, an ideal T beam. Many of the bones are broadened and flat- 

 tened; there is much overlapping, as in the uncinate processes of the 

 ribs; and there is very extensive fusion of adjacent bones, with 

 resultant increase of rigidity. The vertebral column, with the ex- 

 ception of the cervical region, is practically rigid, extensive fusions 

 having taken place between the vertebrae themselves, and between the 

 latter and the bones of the pelvis. The bones of the skuU are almost 

 paper-thin, but are so fused into a unit as to make a practically 

 sutureless brain-box. A large number of bones are lost, especially in 

 the wings and legs, and those that remain are filled with air instead 

 of with bone-marrow. Thus the skeleton of the birds is, among 

 vertebrates, much the lightest for its size, yet the strongest, as it 

 must be to withstand the racking strains incident to flight. 



In a sense the bird is also partially a balloon in that quantities of 

 hot air are carried, not only in the extensive air-sac system, but also 

 inclosed between the body and the feathers and among the innumer- 

 able feather interstices. Nearly half of the contour volume of a bird 

 is air-filled. 



