518 THE TISSUES. 



constituted by the foramen coecum, which is a depression containing 

 fiingiform papillae. The papilla itself is, in structure, to be regarded 

 as a flattened fungiform papilla, except that it contains no elastic 

 tissue; and the wall is a simple elevation of the mucous membrane, 

 with a smooth epithelium, under which its upper border is pro- 

 duced into many rows of simple conical secondary papillae. (Fig. 



Fig. 547. 



Papilla circumvallata of man, in section. A. Proper papilla. B. Wall. a. Epithelium, c. Second- 

 ary papillae, b, b. Nerves of the papilla and of the wall. Magnified about 10 diameters. (KiJlliker.) 



347.) Far more nerves are distributed to these than to the fungi- 

 form papillae; the walls also being abundantly provided with them. 

 Uses of the Lingual Papilla. The filiform papillae are neither the 

 seat of the sense of taste, nor delicate tactile organs; but are the 

 analogues of the lingual spines of animals (Todd and Bowman), 

 and hence aid in mastication, and in protecting the tongue. The 

 two other kinds of papillae subserve the sense of taste, and are also 

 the seat of touch, temperature, &c. The sense of touch is most 

 acute at the point of the tongue where the fungiform papillae are 

 most abundant; the sense of taste at the root of the tongue, pro- 

 bably because the circumvallate papillae possess more nerves in the 

 same space. The nerve-fibres are also finer, or more nearly reduced 

 to isolated axis-fibres. 



Certain pathological appearances of the tongue are easily under- 

 stood from the data just afforded. The gastric furred tongue is 

 produced principally by the growth of the epithelial processes of 

 the filiform papillae, which projecting backwards apparently form 

 & peculiar white coating. If they become longer, so that the pa- 

 pillae measure J to | of an inch, the appearance called the villous 

 tongue, not uncommon in various disorders, is produced. At length, 

 indeed, the tongue may seem to be covered with hairs 4 to 6 lines 

 long. In old people, the tongue may present no papillae at all; 

 they being small and imbedded in the epithelium. Finally, the 

 mucedinous fungi collecting on the papillae filiformes may produce 

 a thick white coat, as shown by Fig. 155. 





