FOOD AND DIGESTION". 



347 



branes in essential points of structure, but contains papilla, more or 

 less peculiar to itself; peculiar, however, in details of structure and ar- 

 rangement, not in their nature. The tongue is beset with numerous 

 mucous follicles and glands. 



The larger papillce of the tongue are thickly set over the anterior 

 two-thirds of its upper surface, or dorsum (fig. 240), and give to it its 

 characteristic roughness. In carnivorous animals, especially those of 

 the cat tribe, the papillae attain a large size, and are developed into 



sharp recurved horny spines. Such papillae 

 cannot be regarded as sensitive, but they en- 

 able the tongue to play the part of a most 

 efficient rasp, as in scraping bones, or of a 

 comb in cleaning fur. Their greater prom- 

 inence than those of the skin is due to their 

 interspaces not being filled up with epithe- 

 lium, as the interspaces of the papillae of 

 the skin are. The papillae of the tongue 

 present several diversities of form; but 



Fig. 241. 



Fig. 242. 



Fig. 241. Section of a mucous gland from the tongue. A, opening of the duct on the free sur- 

 face; C, basement membrane with nuclei; B, flattened epithelial cells lining duct. The duct divides 

 into several branches, which are convoluted and end blindly, being lined throughout by columnar 

 epithelium. D, lumen of one of the tubuli of the gland. X 90. (Klein and Noble Smith.) 



Fig. 242. Vertical section of a circumvallate papilla of the calf. 1 and 3, epithelial layers 

 covering it ; 2, taste goblets ; 4 and 4', duct of serous gland opening out into the pit in which papilla 

 is situated; 5 and 6, nerves ramifying within the papilla. (Engelmann.) 



three principal varieties, differing both in seat and general characters, 

 may usually be distinguished, namely, the (1) circumvallate, the (2) 

 fungiform, and the (3) filiform papillae. Essentially these have all of 

 them the same structure, that is to say, they are all formed by a projec- 

 tion of the mucous membrane, and contain special branches of blood- 

 vessels and nerves. In details of structure, however, they differ consid- 

 erably one from another. 



The surface of each kind is studded by minute conical processes of 

 mucous membrane, which thus form secondary papillae. 



(1.) Circumvallate. These papillae (fig. 242), eight or ten in num- 



