Ixxil 
the parietes of the sac, but, on the contrary, are bent towards 
each other of opposite sides, and can, by a little lateral pres- 
sure, be brought into contact. On the interior of the back 
part of the trachea, exactly opposite the opening in the me- 
dian line, a remarkable prominence, or vertical keel-like pro- 
jection is observed standing forward into the tube, and pre- 
senting a ridge of rounded bifid points or tubercles ; when the 
sides of the opening are approximated, the anterior extremi- 
ties of the six rings come into contact with and are supported 
by this posterior ridge, so that the trachea is there divided into 
two channels, one at either side of this middle line; and thus, 
when the opening into the sac is closed, the respiratory pas- 
sage is maintained free and uninterrupted, while at the same 
time its anterior wall is well supported against any collapse 
into the cavity, and is also enabled to resist the weight or pres- 
sure of the external atmosphere, under the suction influence 
of the inspiratory efforts: this posterior keel-like projection 
extends for some distance on the back part of the trachea, both 
above and below the opening. When the trachea is relaxed 
in the longitudinal direction, the rings are all approximated, 
and those bounding the opening are a little overlapped by that 
above and that below it, and the apposition of the several seg- 
ments is still further secured by the pressure of the superin- 
eumbent soft parts. This cervical air-bag bears no analogy 
to the air-cells disseminated through different parts of the 
bodies of birds; such are formed of cellular tissue, but this 
is an extension of the mucous surface, and has no communi- 
cation with the air-cells, excepting through the trachea and 
lungs. 
The TronGueE (fig. 1) is small, flat, thin, and triangular; 
its surface presents but few papillz, but is studded with innu- 
merable small points, orifices of mucous follicles; its margins are 
neatly fringed with five or six loose, denticulated folds on either 
side, some of which are a quarter of an inch in length. From 
the base of the tongue proceeds backwards a thick semilunar 
