vi FORM AND FUNCTION 81 
tube and tie the windpipe round it with a piece of 
fine string or cotton, then inflate them. The whole 
breast and abdomen will be seen to rise and expand. 
The windpipe should then be tied up and the air-. 
sacks left in a state of inflation. Next the central 
part of the sternum must be got out of the way. Cut 
it longitudinally on either side of the keel from the 
hinder almost to the anterior end. After that remove 
the viscera very carefully, when the extremely delicate 
membrane which forms the sacks may be seen, and 
also the scarlet sponge of the lungs at the bottom 
of them (the bird lying on its back), and the openings 
of the bronchi into the sacks. There are nine sacks 
in all, four on each side, and another pair which have 
run into one. The hindmost or abdominal pair 
are very large, and, when the bird is placed upon its 
back, lie over the kidneys and under the intestines, 
extending far back behind the lungs. In _ front 
of them are the posterior thoracic, and next to 
them the anterior thoracic, sacks. Then comes the 
interclavicular sack formed of two which have coa- 
lesced. The middle part of this can easily be seen 
in the angle between the clavicles or wishbones, 
but it also runs out on either side to the shoulder 
bones (Fig. 23). The cervical sacks are very small 
and lie at the base of the neck. 
As yet I have only-described the minimum of air 
sacks common to all birds: in many species there are 
air cavities in the bones, sometimes extending even to 
the very extremities of the limbs: in some they are 
found under the skin also, and even in some of the 
feathers and between the muscles. In a young bird 
G 
