vi FORM AND FUNCTION Si 



tube and tic 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 



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