THE ALIMENTARY TUACT. 



277 



a long-itudinal slit about 3 mm. in length. The mucous mem- 

 brane to either side of the tongue is only slightly folded, and 

 a little in front of the angle of the mouth is depressed into 

 an aperture (6), opening into the vocal sac. This opening is sur- 

 rounded by small radiating folds of mucous membrane, and is oval 

 in shape. 



Towards the oesophagus the mu- 

 cous membrane of both the roof 

 and the floor of the mouth is thrown 

 into numerous longitudinal folds. 



b. The minute structure of the 



several parts. 



(i) The mucous membrane of 

 the mouth. At the junction of the 

 skin and the mucous membrane 

 there is a gradual transition from 

 the stratified epithelium of the 

 skin to a single layer of colum- 

 nar epithelium. On the floor 

 of the mouth this condition is 

 reached at the tongue ; on the 

 roof the transition takes place 

 more quickly. The columnar 

 epithelium of the mouth (that of 

 the tongue is excluded from this 

 description) is ciliated ; scattered cells, which are not ciliated, are 

 found here and there irregularly, but are not numerous. The 

 cylindrical cells are very finely granular in their upper parts, 

 clearer in the middle portion, more coarsely and darkly granular in 

 their deeper portions ; each cell is possessed of a large, well-defined, 

 oval nucleus, which contains one or sometimes two nucleoli. The 

 non-ciliated cells usually present a sharply differentiated, structure- 

 less, hyaline, free border (Schultze). The epithelium towards the 

 margin of the mouth, where it is stratified, is also ciliated. 

 | Goblet-cells occur in every part of the epithelium, whether cili- 

 ' ated or non-ciliated, and their forms vary very greatly. 



[The submucous layer is a fibrous connective-tissue matrix, 

 very rich in nerves and blood-vessels. The capillaries are arranged 

 in small, somewhat polygonal meshes (Fig. 180), and are peculiar, 

 with those of the anterior part of the oesophagus, in having small 



The floor of the mouth. 

 A" Muscles. 

 L Opening to larynx. 

 Jf Mandible. 



S Opening to voice-sac (in males only) 

 Sm Chin. 

 7. Tongue. 



X 1 Left cornn of bifid tongue. 

 + Folds opposite hinder border of the 

 hvoid. ' 



