FLIGHT. 239 



the muscles of the wings may have sufficient space 

 and firmness for their attachment. Both these objects 

 are attained by a modification of the apparatus of 

 breathing-. The lungs are highly vascular and 

 spongy, but they are not distended with air. The 

 air is drawn through their substance into the large 

 cavity, common to the chest and abdomen ; whilst the 

 great office of decarbonization of the blood is securely 

 performed, advantage is taken to let the air into all 

 the cavities, even into those of the bones. 



" The weight of the body being a necessary con- 

 comitant of muscular strength, we see why birds, 

 by reason of their lightness, as well as by the con- 

 formation of their skeleton, walk badly. And, on the 

 other hand, in observing how this lightness is adapted 

 for flight, it is remarkable how small an addition 

 to their body will prevent them rising on the wing. 

 If the griffon-vulture be frightened after his repast, 

 he must disgorge before he flies ; and the condor, in 

 the same circumstances, is taken by the Indians, like 

 a quadruped, by throwing the lasso over it. It is 

 interesting to notice the relations of great functions 

 in the animal economy : birds are oviparous, because 

 they never could have risen on the wing had they been 

 viviparous ; if the full stomach of a carnivorous bird 

 retard its flight, we perceive that it could not have 

 carried its young. The light body, the quill-feathers, 

 the bill, and the laying of eggs, are all necessarily 

 connected. 



" As every one must have observed, the breast- 

 bone of birds extends the whole length of the body ; 

 and owing to this extension, a lesser degree of motion 

 suffices to respiration ; so that a greater surface, 

 necessary for the lodgement and attachment of the 

 muscles of the wings, is obtained, whilst that surface is 

 less disturbed by the action of breathing, and is more 

 steady. Another peculiarity of the skeleton of the 



2 A3 



