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 lings 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- 
