Ixxii 



the paiietes 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 eiforts : 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- 

 cumbent soft parts. This cervical air-bag bears no analogy 

 to the air-cells disseminate^ 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 TONGUE (fig. 1) is small, flat, thin, and triangular ; 

 its surface presents but few papillae, but is studded with innu- 

 merable small points, orifices of mucous follicles; its margins are 

 neatlyfringed 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 



