548 THE SENSE OF TASTE. 



The tongue is a muscular organ covered by mucous mem- 

 brane ; the latter resembling other mucous membranes (p. 

 316) in essential points of structure, but containing certain 

 parts, the papillae, more or less peculiar to itself; peculiar, 

 however, in details of structure and arrangement, not in their 

 nature. The tongue is beset with numerous mucous follicles 

 and glands. Its use in relation to mastication and deglutition 

 has already been considered (p. 213). 



Besides other functions, the mucous membrane of the tongue 

 serves as a groundwork for the ramification of the abundant 

 bloodvessels and nerves which the tongue receives, and affords 

 insertion to the extremities of the muscular fibres of which the 

 chief substance of the organ is composed. 



The larger papillae of the tongue are thickly set over the an- 

 terior two-thirds of its upper surface, or dor sum (Fig. 202), and 

 give to it its characteristic roughness. Their greater promi- 

 nence than those of the skin is due to their interspaces not being 

 filled up with epithelium, as the interspaces of the papillae of 

 the skin are. The papillae of the tongue present several di- 

 versities of form ; but three principal varieties, differing both 

 in seat and general characters, may usually be distinguished, 

 namely, the circumvallate or calyciform, the funpiform, and the 

 filiform papillae. Essentially these have all of them the same 

 structure, that is to say, they are all formed by a projection 

 of the mucous membrane, and contain special branches of 

 bloodvessels and nerves. In details of structure, however, they 

 differ considerably one from another. 



All the three varieties of papillae just described have been 

 commonly regarded as simple processes, like the papillae of the 

 skin; but Todd and Bowman have shown that the surface of 

 each kind is studded by minute conical processes of mucous 

 membrane, which thus form secondary papillae. These secon- 

 dary papillae also occur over most other parts of the tongue, not 

 occupied by the compound papillae, and extend for some dis- 

 tance behind the papillae circumvallatae. The mucous mem- 

 brane immediately in front of the epiglottis is, however, free 

 from them. They are commonly buried beneath the epithe- 

 lium ; hence they had been previously overlooked. 



Circumvallate or Calyciform Papillce. These papillae (Fig. 

 203), eight or ten in number, are situate in two V-shaped 

 lines at the base of the tongue (1, 1, Fig. 202). They are 

 circular elevations, from o^th to y^th of an inch wide, each 

 with a central depression, and surrounded by a circular fissure, 

 at the outside of which again is a slightly elevated ring, both 

 the central elevation and the ring being formed of close-set 

 simple papillae (Fig. 203). 



