44 



MORPHOLOGY OF THE VERTEBRATE BODY 



may, therefore, be said to possess a mouth cavity comparable 

 with that occurring at an early stage in the development of air- 

 breathing vertebrates, the mouth region of these higher verte- 

 brates being, as it were, a " two-story " affair derived in the 

 manner indicated. 



Another point of difference is the attachment of the tongue, 

 which extends backward and not forward into the mouth of the 



n.c. 



Fig. 24. — Median section of human head for comparison with that of frog. 



e, epiglottis; es, esophagus; E.t., Eustachian tube; /.s.. frontal sinus; yl, glottis; h.p., 

 hard palate; n, nostril; n.c, nasal cavity; s.p., soft palate; sp.cL, spinal column; sp.s., 

 sphenoidal sinus; t, tongue; tb, turbinal; tr, trachea; v.c, vocal cords. 



frog, in contrast with the condition existing in most of the famihar 

 vertebrate animals. This difference, however, is easily explained. 

 The tongue is a thickened portion of the floor of the mouth. In 

 most cases where it is well developed, it grows out anteriorly, but 

 in a few instances, including the frogs and toads, it extends pos- 

 teriorly. Correlated with the posterior growth in these animals 

 is their habit of suddenly " flopping " the tongue outward in the 



