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 
