AIR-SACKS 



are present in great numbers between the muscles and the roots of 

 the feathers. These birds when inflated and pricked emit a 

 peculiar hissing noise through the skin. It is well known that a 

 bird which has its humerus shattered by shot can for some time 

 breathe, although its beak and nostrils be tightly closed, and thus 

 be submitted to unnecessary excruciating pain. Compression of 

 the thorax and abdomen suffocates a wounded bifd better than 

 strangulation. 



II. The naso-pharyngeal or tympanic system of air -sacs is 

 restricted to the head, extending chiefly into the occipital, frontal, 

 parietal, quadi'ate, and mandibular bones. To this system belong 

 the Eustachian tubes (see Ear and Skull), the tympanic, and other 

 cavities which communicate with the nose. The most curious 

 dilatation belonging to this system is the crop-like pouch of the 

 Adjutant. This sac communicates in Leptoptilus crumenifer with a 

 large cavity below the orbit and the pterygoid bone on the left side 

 of the basis cranii, opening directly into the nasal cavity and extend- 

 ing like a hei'nia into a loose fold of integument, the pouch being 

 divided into two by a vertical membrane which descends to the 

 level of the eighth cervical vertebra. 



Another inflatable sac is the gular pouch of Bustards. It seems 

 to be developed only in adult males, reaching its gi'eatest size 

 during the breeding season, and again shrivelling up during the rest 

 of the year. Its opening is a 1-shaped slit in front of the frenulum 

 of the tongue and below this organ ; the opening can be closed by 

 muscles, and leads into a large, glandless blind sac (about 8-10 inches 

 long, with half the width), which is a dilatation of the frenulum 

 and hangs down between the throat and the skin of the front of 

 the neck. It seems to be an entirely sexual ornament, inflating the 

 skin, and containing neither water nor food. 



A similar homologous structure exists in the male of Blzmra 

 lobata, as a little pouch between the two halves of the frenulum, 

 with a roundish opening, but apparently not extending into or 

 inflating the outer cutaneous wattle or fold underneath the 

 mandibles. 



Lastly, the tracheal pouch of the Emeu may be mentioned. It 

 is a large unpaired hernia-like sac of the tracheal walls, communi- 

 cating with the trachea through a longitudinal slit on the ventral 

 side, an individually- varying number of from five to fourteen car- 

 tilaginous rings being known to be deficient in the middle line. In 

 the embryos this deficiency is already shewn, but the pouch is 

 developed much later, and attains its full size in the adults of both 

 sexes. This organ seems to act as a resounding bag to the joeculiar 

 drumming noise made by the adult birds. 



The function of all these air-sacs has been the subject of many 

 controversies. Some are undoubtedly subservient to sexual orna- 



