158 INTESTINAL MUCOUS MEMBRANE. 



varieties, the inner or proper surface of the papilla 

 is studded with secondary papillae, in the shape of 

 slender processes of variable length (PL XXV. fig. III. 

 2 ; fig. IV. 2). As for the epithelium, it adapts itself 

 accurately to the subjacent membrane, presenting a 

 free surface which differs in appearance according as it 

 corresponds to the localities occupied by the circurn- 

 vallate and fungiform papillae, or to those of the fili- 

 form variety ; in the first case it is perfectly smooth 

 (fig. III. 3), but in the latter it presents long and 

 delicate filaments, variable in length, and identical in 

 shape with the secondary papillae, the surface of 

 which they cover (fig. IV. 3, 4). It is this epithe- 

 lium of the filiform papillae which in some animals 

 assumes a horny character, and thus constitutes a 

 prehensile organ ; in the pike its structure is identi- 

 cal with that of its teeth. The epithelium of the 

 tongue is identical with that of the lips and cheeks, 

 both as regards the form and disposition of its cells. 



The filiform papillae differ from both of the other 

 varieties, not only in their epithelial aspect, but also 

 in their relations to the nervous system ; thus the 

 nervous filaments which penetrate them are few in 

 number, and they do not reach their secondary 

 papillae. The circum vail ate and fungiform papillae, 

 on the contrary, are relatively rich in nerve fibres, 

 and they can be traced readily into their secondary 

 papillae, where they seem to terminate by free extre- 

 mities ; Kolliker has even demonstrated the presence 

 of tactile corpuscles in the fungiform papillae of the 

 tip of the tongue. 



The blood-vessels which they receive are distri- 



