Ixxvi 



in mucous membrane, and engaged in the edge of the tracheal 

 opening ; the posterior cornu meets the corresponding cornu 

 from the other side, in the posterior indentation, and both 

 project forwards from the posterior wall of the trachea, and 

 present the appearance internally already noticed. These pos- 

 terior cornua, from the right and left crescentic cartilages, 

 are, in some, connected together by a narrow and thin carti- 

 laginous structure ; in others, by a narrow line of dense cellu- 

 lar tissue ; so that we may regard this portion of the trachea 

 as composed not of six cartilaginous rings, open in front, but 

 of twelve crescentic cartilages, six on each side ; their ante- 

 rior cornua either bounding the sides of the opening when 

 this is open, or in contact, when it is closed ; their posterior 

 cornua bent forwards, so as to form the keel-like prominence 

 internally, which will come into contact with and support the 

 anterior cornua when these are approximated and the orifice 

 closed, and the tracheal canal thereby divided into two lateral 

 tubes. 



From an examination of the several parts concerned in 

 this curious apparatus, and from observing the animal during 

 life, I am led to infer that it possesses the voluntary power of 

 not only expanding this air-bag, but also of retaining it in 

 that state for an indefinite time, without any continued mus- 

 cular exertion, and that it can either rapidly or slowly con- 

 tract, or empty it, and perfectly close its communication with 

 the trachea. It has been remarked that on some days it is 

 not dilated ; on others it is frequently expanded, and as fre- 

 quently contracted, in a few moments, or retained in a dis- 

 tended state for a considerable time ; and on some occasions 

 it remains in that condition when the bird is at rest, or appa- 

 rently asleep. When about to fill it, he raises and slightly 

 extends the neck, and darts it a little forward; little or no 

 muscular exertion is apparent, and the bag swells out, most 

 probably by an expiratory effort, the glottis being previously 

 closed, and the muscular wall of the sac being relaxed, so as 



