THE BOOK OF MIGRATORY BIRDS 7 1 



blowing with a little force into the windpipe, by which 

 means the belly may be blown up to a considerable size, 

 a circumstance which would not occur in other animals." 



Some remarks by Sir Charles Bell on the subject of 

 buoyancy are well worth reproducing. * 'First," he says, 

 "it is necessary that birds, as they are buoyed in the air, 

 be specifically lighter ; secondly, the circumference of their 

 thorax must be extended and the motions of their ribs 

 limited, that 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 weight of the body being a necessary concomitant 

 of muscular strength, we see why birds, by reason of their 

 lightness, as well as by the conformation 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. 



"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 everyone must have observed, the breastbone of 

 birds extends the whole length of the body, and, owing 

 to this extension, a lesser degree of motion suffices for 

 respiration ; so that a greater surface, necessary for the 

 lodgment 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. The vertebrae of 

 the back being fixed in birds, and the pelvis reaching 

 high, there is no motion in the body ; indeed, if there were 

 it would be interrupted by the sternum. We cannot but 

 admire, therefore, the composition of the neck and head, 



