SEC. 3. THE STRUCTURE OF THE ORGANS OF TASTE. 



862. Sensations of taste, like those of sight, hearing and 

 smell, are brought about by means of special nerve fibres whose 

 endings are connected with a modified epithelium ; but these 

 special nerve fibres do not form a nerve devoted exclusively to 

 taste as the optic nerve is to sight, the olfactory nerve to smell, 

 and though less distinctly, the auditory nerve to hearing ; they 

 run in company with fibres of other kinds in one or other or both 

 of two cranial nerves, in the glossopharyngeal nerve and in the 

 lingual branch of the fifth nerve. In this respect the sense of 

 taste shews a tendency to a more general distribution similar to 

 that of touch ; and in the lower animals this tendency is still 

 more strongly marked. Moreover, though as we shall see, a 

 modified epithelium, subserving taste, does exist in the lining 

 of the mouth, it is not so clear, as in the case of sight, hearing 

 and smell, that the development of gustatory sensations is ex- 

 clusively dependent on this modified epithelium ; there are reasons 

 for thinking that taste may be exercised by parts of the mouth 

 where no such distinctly modified epithelium exists. 



Although the lining membrane of the mouth is generally 

 spoken of as a " mucous membrane," its histological plan is 

 identical with that of the skin. The epithelium overlying the 

 dermis is a stratified epithelium, consisting of many tiers of cells, 

 in which we may distinguish a lower malpighian layer and an upper 

 corneous layer. One of the chief differences between the epithelium 

 of the mouth and the epidermis of the skin is, that in the former the 

 stratum granulosum and stratum lucidum ( 434) so character- 

 istic of the latter, are absent, or present in a few places only, being 

 replaced by a layer in which the polygonal prickle cells of the 

 malpighian layer, losing their prickles and becoming flatter, are 

 gradually transformed into the cells of the corneous layer. 

 Another difference is that the uppermost cells of the corneous 

 layer, though transformed into flat scales, still retain a con- 

 spicuous nucleus, and are not so wholly corneous as the cor- 

 responding scales of the skin. The differences appear to be 

 connected with the fact that the epithelium of the mouth is 



