1 8 A HISTORY OF BIRDS 



large pectoral muscles, and the body and keel of the 



sternum. . , . . 



The third and fourth pair are the "anterior and posterior 

 intermediate sacs," and are enclosed within the thoracic cavity, 

 extending backwards more or less far into the abdominal cavity. 



The fifth pair form the "abdo- 

 minal sacs ". Of large size, they are 

 continued backwards to the end of 

 the abdominal cavity. 



This elaborate system of air- 

 chambers is further complicated in 

 many birds by an extensive system 

 of pneumatic cells extending between 

 the muscles, and between the muscles 

 and the skin,so that the bird is abso- 

 lutely encased in a layer of air en- 

 closed between the skin and the 

 body. This emphysematous condi- - 

 tion is most perfectly developed in 

 birds such as the Gannets, Screamers 

 and Hombills. 



The Naso-pharyngeal System of 

 Air-sacs 



But there is yet another system 

 of air-cells to be mentioned, which 

 is known as the " tympanic " or 

 " naso-pharyngeal system ". While 

 in the majority of birds this is con- 

 cerned merely with the supply of air 

 to the bones of the skull, in the 

 Adjutant Storks it gives rise to a 

 very remarkable pouch which runs 

 down the front of the neck as far 

 as its middle. It can be inflated and 

 deflated at pleasure, communicating 

 as it does with the nasal chamber 

 through an aperture in the floor of 

 the orbit. 



-pouches of the Great Bustard and the 

 here. That of the first-named forms 



L.A.U.S. 



III. 6. — Dissection Showing 

 THE Lungs and Air-sacs of 

 A Bird 



Pb. s. = Pre-bronchial sac. 

 Ax. = Axillary sac bounded ex- 

 ternally by the breast-muscles, 

 seen here in section. S. s. = 

 Partition dividing anterior inter- 

 mediate sac (A.i.s.) from the 

 sub-bronchial sac. P.i.s. = Pos- 

 terior intermediate sac. O.s. = 

 Oblique septum. H.s. = Hori- 

 zontal septum. L. Ah. s. = 

 Left abdominal sac. H. = 

 Heart. G. = Gizzard. L. = 

 Liver. In. = Intestine. (After 

 Strasser.) 



The extraordinary air 

 Emu may be mentioned 



