8o THE STRUCTURE AND LIFE OF BIRDS chap. 



more work, in proportion to the size of his body, out 

 of his exceedingly small and light lungs than a man 

 can out of his far heavier apparatus. Any one who 



ICS 



Fig. 23 — Diagram after Heider. Air-sacks excepting the cervical. The lungs are 

 shaded dark. Ahs, abdominal sack ; as, anterior thoracic ; B, entrance of bronchial 

 membrane; H, humerus; ics, interclavicular sack, surrounding trachea; and 1, 2, 

 3, 4, its extensions ; 2 opens between the pectoral muscles ; rs, posterior thoracic 

 sack ; t, trachea. See Fig. 25. 



wishes to see the air-sacks — and to see them is much 

 better than only to read of them — should take some 

 bird of moderate size, such as a pigeon, cut through 

 the windpipe somewhere in the neck, insert a blowing- 



