174 The Bird 



air between the muscles and the skin, and when we 

 handle a bird thus aerated the skin crackles under our 

 touch. 



The lungs and air-sacs send off tiny membranous 

 tubes which enter the bones of the limbs and skull and 

 sometimes even the small bones of the wings and toes, 

 which are hollow and thus filled with air. It seems in- 

 credible, but nevertheless it is true that the connection 

 between the lungs and the upper arm-bone of a bird is 

 so substantial that a bird which has had its wing broken 

 with shot is able to breathe through the splintered end 

 of this hollow bone when its windpipe is completely 

 choked with blood. 



We may compare the body of a bird to a submarine 

 boat with many water-tight compartments, and as such 

 a vessel is made buoyant by admitting air to these bulk- 

 heads, so a swimming bird may float high out of water 

 by inflating its sacs and filling its bone-cavities with air. 

 Conversely, when we see a grebe slowly and mysteriously 

 submerge its body, we conclude that it has but emptied 

 its lung auxiliaries. 



We now come to the most important part of the re- 

 spiratory system, where the blood and the air come into 

 closest contact and exchange gases, the oxygen of the 

 air vitalizing the entire body. If we follow the two 

 bronchial tubes after they leave the syrinx, we shall find 

 that each enters a lung, and passes through it, giving 

 off a number of side branches which open into the vari- 

 ous air-sacs. The lungs are not elastic and, instead of 

 lying freely in .the body, are flattened against the back- 



