IV 



THE DIGESTIVE SYSTEM 



In the fowl the Hps and cheeks are replaced 

 by the beak, an area of dense and horny skin 

 lying over the incisive bones and the mandible, 

 which serve as a bony foundation. Owing to 

 the absence of anything homologous with the 

 soft palate of mammals, the posterior limit of 

 the mouth cavity of birds is not exactly defined ; 

 but for convenience may be taken as indicated 

 by a prominent row of papillae on the tongue 

 and the last row of papillae on the hard 

 palate (Figs. 10 and 11). The true embryo- 

 logical limit appears to be slightly farther back ; 

 or, in other words, approximately on a level 

 with the opening into the larynx. The diffi- 

 culty of giving an exact definition is rendered 

 all the greater by the assertion of embryo- 

 logists that the tongue is developmentally a 

 pharyngeal structure, and, therefore, does not 

 strictly belong to the mouth. 



40 



