322 THE HARMONIES OP NATURE. 



but at every impulse of their flight such a motion would de- 

 prive the whole limb of its resistance to the air ; or it would 

 require the constant exertion of such a degree of antagonising 

 muscular force to prevent it, as would be incompatible with the 

 essential structure of these organs of flight. 



As we have seen in the preceding chapter, the natatorial birds 

 have a large gland situated at the extremity of the back, with 

 whose unctuous secretion they lubricate their feathers, and thus 

 render them impermeable to the wet. A similar protection has 

 also been afforded to the bats in the oily secretion of their wings, 

 so that in damp weather the rain, which otherwise would very 

 much impede their flight, drops from them as from a weather- 

 proof mantle. 



But it is not only by their winglike arms that the bats are 

 admirably formed for flight the whole structure of their body 

 harmonises as well with their peculiar mode of life, as the long- 

 tapering shape of the seals or whales with their aquatic habits. 

 All their bones, not merely those of the arms, are extremely thin 

 and slight ; the breastbone is provided, like that of the birds, 

 with a keel, serving for the attachment of the powerful muscles 

 which perform the movements of flight ; the head is singularly 

 diminutive and the neck short, so that the centre of gravity falls 

 below the wings. In the genus Nycteris the specific weight of 

 the body, and consequently the labour of flying, is still more 

 diminished by the curious faculty possessed by these animals of 

 inflating the subcutaneous tissue with air. The skin adheres to 

 the body only at certain points ; it is therefore susceptible of 

 being raised from the surface on the back, as well as on the 

 under-parts. These large spaces are filled with air at the will 

 of the Nycteris, by means of large cheek-pouches, which are 

 pierced at the bottom, and thus communicate with the subcu- 

 taneous spaces just mentioned. When the animal therefore 

 wishes to inflate its skin, it inspires, closes the nostrils, and then, 

 as it contracts the cavity of the chest, the air is forced through the 

 openings in the cheek -pouches, under the skin, from whence it is 

 prevented from returning by means of muscular contraction, 

 and by large valves on the neck and back. By this curious 

 mechanism, the bat has the power of so completely blowing up 

 the spaces under the skin as to give the idea of a small balloon, 

 with wings, a head, and feet. In Johnson's well-known tale of 



