80 THE STRUCTURE AND LIFE OF BIRDS cnap. 
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 
Fic. 23—Diagram after Heider. Air-sacks excepting the cervical. The lungs are 
shaded dark. abs, abdominal sack ; As, anterior choradie 8B, entrance of bronchial 
membrane ; H, humerus; 1cs, interclavicular sack, surrounding trachea; and 1, 2, 
3, 4, its extensions ; 2 ae between the pectoral muscles; Ps, 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- 
