250 



AVES. 



by a pleural sac, but are attached to the dorsal wall of the body cavity 

 by cellular tissue, and sunk in the interspaces between the ribs at 

 the sides of the vertebral column. The behaviour of the bronchial 

 tubes and the structure of the finer respiratory air-spaces of the lungs 

 present essential differences from those of the Mammalia. The large 



air-sacs are diverticula of the 

 lungs (fig. 658); they have 

 a fairly constant arrange- 

 ment, extending forwards 

 between the clavicles (peri- 

 tracheal or interclavicular 

 air-sac), and also into the 

 anterior and lateral regions 

 of the thorax (thoracic air- 

 sacs), and backwards among 

 the viscera, into the pelvic 

 region of the abdominal 

 cavity (abdominal air-sacs). 

 The abdominal sacs lead into 

 the cavities of the femora 

 and pelvic bones, while the 

 smaller anterior sacs are pro- 

 longed into the air-spaces of 

 the bones of the arm, and 

 into those of the skin, which 

 are sometimes, especially in 

 the large swimming Birds, 

 which fly well (Sula, Peleca- 

 nus), so numerous that the 

 skin emits a crackling sound 

 when touched (maintenance 

 of temperature, reduction of 

 specific gravity, air reservoirs 

 for respiration). With such 

 arrangements combined with 

 the rudimentary form of the 

 diaphragm already mentioned, and the peculiar structure of the 

 thorax, the mechanism of respiration must be quite unlike that of 

 the Mammalia. The dilatation of the thoracic framework, which also 

 encloses the abdominal cavity, is the result of the extension of the 

 sternocostal bones and the consequent increase in the distance 



FIG. 658. Lungs and air-sacs of the pigeon (dia- 

 grammatic, after C. Heider). 2V, trachea; P, 

 lungs ; Lp, peritracheal air-sac with its diver- 

 ticula (Lh and Lni) into the humerus (H) and 

 between the pectoral muscles; C, their con- 

 nection with the sternal air-spaces ; Lth, thoracic 

 air-sacs ; La, abdominal air-sacs. 



