THE LIPS AND BEAK. 151 



epithelial cell. In shape it is a hollow cone, proclnced at its 

 apex into a spoon-shaped process, notched at its free edge. 

 These horny epithelial teeth are easily rubbed oif during use, 

 and are speedily replaced by other similar ones formed beneath 

 them. Bach tooth is in fact the top member of a column of 

 specially modified epithelial cells, imbedded in the general 

 epithelium of the lip. In each column the deepest cells are 

 ordinary epithelial cells, scarcely distinguishable from those in 

 which they are imbedded : the succeeding cells of the column, 

 nearer the surface, become first flattened, then cup-shaped, and 

 finally conical, the apex of the cone fitting into the cavity of 

 the cell next above it. 



The deeper cells of the column are soft, and have distinct 

 nuclei ; nearer the surface the cells have their outer layers con- 

 verted into homy matter, while their shape gradually approaches 

 that of the fully formed teeth. The nucleus becomes less dis- 

 tinct, and finally disappears, as the cornification extends deeper 

 and deeper into the substance of the cell. 



Each tooth is thus formed by cornification of a single epi- 

 thelial cell, which commences its career in the deeper layer of the 

 epidermis, at the base of the column, and gradually approaches 

 the surface through loss of the teeth above it, acquiring, as it 

 does so, the characters of the fully formed tooth. On reaching 

 the surface it comes into functional use for a time, and then in 

 its turn becomes rubbed off" and lost. 



3. The Beak. 



The beak consists of the two jaws, upper and lower, and is 

 in shape not unlike that of a bird or turtle (Figs. 65 and 71). 

 Each jaw is a strong, curved band of cornified epithelium, sup- 

 ported at its base by the labial cartilages (Fig. 90, LU, Ll), and 

 ending at its free surface in a sharp biting edge. The upper or 

 maxillary jaw (Fig. 65) is longer and less sharply curved ; the 

 lower or mandibular jaw, which bites behind the upper jaw, is 

 shorter, stronger, and almost horse-shoe shaped in outline. 



The minute structure of the two jaws is the same, each con- 

 sisting of modified epithelial cells. The cutting edge of the jaw 

 is formed by a row of horny teeth, very similar to those of the 

 labial rows, but placed so closely side by side as to form a con- 

 tinuous blade. Each of these teeth is, as in the case of the labial 



