380 MANUAL OF HISTOLOGY. 



times be seen also with double contours, which wind, during 

 their course, into a coil in the superficial layers of the mucous 

 membrane. 



THE TONGUE. 



Although the mucous membrane of the tongue is, in the 

 general details of its construction, similar to that of the rest of 

 the buccal cavity, it nevertheless presents some striking pecu- 

 liarities, mainly due to the configuration of its upper surface. 

 This is covered by many closely aggregated prominences of 

 the mucous membrane the lingual papillce which give it a 

 roughened, fungoid appearance. Upon the under surface of 

 the tongue the papillae are absent, but the mucous membrane 

 here contains a large number of follicular glands. The lateral 

 edges of the tongue are here and there covered with lingual pa- 

 pillae, which are often arranged in rows, and toward the base 

 of the tongue are replaced by the so-called fimbrice linguce. 

 Besides simple papillae, analogous to those of the skin, the lin- 

 gual mucosa is studded with three distinct varieties of com- 

 pound papillae the filiform, the fungiform, and the circum- 

 vallate. These are distinguished from the ordinary papillae of 

 the mucous membrane, not only by their large size and their 

 peculiar shapes, but also by their complicated structure, by the 

 arrangement of their secondary papillae, and the conditions of 

 their epithelial coverings. Between these three forms are 

 several intermediary ones. The filiform papillce are found all 

 over the dorsum of the tongue, anterior to the line of the cir- 

 cumvallate papillae. Not only in different individuals, but 

 also in the same tongue, there are marked variations in their 

 form. At the tip and lateral edges of the tongue they are 

 always smaller, and their filaments are wanting or merely ru- 

 dimentary. Toward the centre of the tongue they gradually 

 become larger and more abundant, and attain their highest 

 development in the angle made by the circumvallate papillae. 



Their shape is that of a truncated cone, which has at its 

 free extremity a central hollow or depression, around which is 

 arranged, in a circular manner, a collection of thread-like pro- 

 jections, or secondary papillae. Like the rest of the mucous 

 membrane of the tongue, they are covered with stratified pave- 

 ment-epithelium. In the secondary papillae of the larger fili- 



