BIRDS 163 



large air pouch in the front of the neck which communicates 

 with the trachea by a narrow upright slit. The pouch was 

 found by Dr. Murie l and other anatomists to be present in 

 both sexes. But Mr. Bartlett observed that the lower part 

 of the neck was much more inflated in the female than in 

 the male when the call or voice was uttered. According to 

 Dr. Murie the female forcibly dilates the sac by closing the 

 glottis and forcing air through the tracheal slit ; and he 

 thought the sound was produced by the rush of air past the 

 opening of the pouch, as when air is blown over the open 

 bung-hole of a cask. 



On the whole, it seems possible that the sound may be in 

 this case of secondary importance, and that the primary fact 

 is the inflation of the neck. We know that so many 

 animals, and birds in particular, puff and swell themselves 

 out under the influence of sexual excitement, e.g. the 

 bustard. If sound were the object, why should it not be 

 produced in the usual way in the syrinx, or lower larynx, 

 which is situated not at the glottis but below the opening of 

 the air pouch ? Whether the bird is making an effort to 

 produce a sound or merely to swell its throat, it is certain 

 that it does close its glottis and then violently force air into 

 its trachea. The trachea is therefore subjected to the pres- 

 sure of compressed air, and thus there exists a mechanical 

 cause corresponding to the existence of a dilatation of the wall 

 of the tube at a particular point. The mechanical cause 

 appears, like the structural modification, to exist in both 

 sexes, but to a greater degree in the female. As is usually 

 the case with organs associated with sexual activity, the 

 pouch is much less developed in young and immature 

 individuals. 



It may be objected that it is difficult to understand why 

 tracheal rings should be burst by internal pressure in such a 



1 Proc. Zool. Soc. 1867. 



