THE COELOM, DIGESTIVE, AND RESPIRATORY SYSTEMS 177 



of imperfect development. In the roof of the mouth cavity, dorsal to the palatal 

 folds and concealed by them, are the posterior nares. Locate them by bending 

 aside or cutting away the palatal folds. Probe into the anterior nares and note 

 that the probe emerges through the posterior nares. The floor of the mouth 

 cavity is occupied by the pointed tongue, whose posterior free border is fimbriated 

 and terminates in a point on each side. The tongue of birds is not very muscular. 

 Numerous glands open into the oral cavity in birds but are too small to study 

 in gross dissection. 



b) Pharynx: Note that, as in all adult vertebrates above urodeles, gill slits 

 are absent from the lateral walls of the pharynx. In the roof of the pharyngeal 

 cavity just posterior to the caudal ends of the palatal folds is a median aperture, 

 the opening of the paired auditory tubes. Each auditory tube extends from this 

 opening to the cavity of the middle ear; tube and cavity represent in part an 

 evagination from the first visceral pouch. In birds unlike other vertebrates the 

 two auditory tubes unite to one at the point of communication with the pharynx. 

 Posterior to this opening the roof of the pharynx bears a pair of folds with fim- 

 briated borders, which hang down like a curtain into the pharyngeal cavity. 

 These folds constitute the soft palate. In the floor of the pharynx, immediately 

 posterior to the caudal end of the tongue, is a hardened elevation, the laryngeal 

 prominence, bearing in its center an elongated opening, the glottis. The margins 

 of the glottis are also fimbriated, and immediately posterior to the glottis on 

 each side is a fringed fold. In the walls of the glottis the supporting laryngeal 

 cartilages are readily felt. 



Make a drawing of the oral and pharyngeal cavities. 



2. The hyoid apparatus, the larynx, the trachea, and the esophagus. — 

 Make a median ventral longitudinal incision in the skin of the neck from the 

 throat to the anterior end of the sternum. Deflect the skin on each side of the 

 incision. The trachea or windpipe, a tube with walls stiffened by rings of carti- 

 lage, is immediately exposed. Dorsal to it or to one side of it is the soft 

 esophagus. 



Trace the trachea forward to the glottis, cleaning away the muscles which 

 cover its anterior end. At the same time a cut may be made to the sides of the 

 tongue so that the tongue may be pulled down ventrally from the mouth cavity. 

 The hyoid apparatus may now be studied. It consists of remnants of the hyoid 

 (second) and third gill arches. It is composed of three median elements, arranged 

 in a longitudinal series, and two pairs of horns or comua. The most anterior 

 of the three median pieces is the entoglossal cartilage. It is situated inside of 

 the tongue and may be revealed by dissecting off the covering membrane of the 

 tongue. It represents the two fused ceratohyals. From its posterior end pro- 

 jects posteriorly on each side a small cartilage which occupies the caudal point 

 of the tongue already noted. These two cartilages constitute the anterior 

 horns of the hyoid and consist of the free ends of the two ceratohyals whose 



