1240 THE SENSE OF TASTE. 



nerve roots, rather than one of the fifth nerve, many of the pathological 

 cases recorded receive an equally satisfactory explanation. 



As the case is one that cannot be determined by experiments on 

 lower animals, the positive evidence of the retention of taste in man, 

 after section of the fifth nerve roots, is of great interest. 



It is generally admitted that the fibres of taste from the back of the 

 tongue pass up the branches of distribution of the glosso-pharyngeal 

 nerve. It is not certain whether they ascend to the brain by the same 

 nerve, and many believe that they pass by a rather circuitous path to 

 the fifth nerve. It is frequently stated that disease involving the trunk 

 of the glosso-pharyngeal nerve never abolishes taste, though cases have 

 been recorded in which it is maintained that taste was lost under these 

 conditions. 1 It is supposed that the taste fibres ascend in the tympanic 

 branch (nerve of Jacobson), and reach the fifth through the small super- 

 ficial petrosal, and through branches of the tympanic plexus. In support 

 of this view, it is urged that disease of the fifth nerve is often followed 

 by loss of taste on the back of the tongue, and that disease of the middle 

 ear, which is especially liable to affect Jacobson's nerve, may be followed 

 by a similar result. On the other hand, in direct contradiction to this 

 view, we have the observations of MacTiffany, Krause, and Lynn Thomas 

 above mentioned, as well as the embryological evidence, that the nerve 

 of Jacobson is developed as a branch of the glosso-pharyngeal, and so 

 would not be expected to carry impulses from that nerve into the 

 course of the fifth. 2 



The tongue may be paralysed as to its nerves of general sensibility, 

 whilst the sense of taste may remain normal, and in other cases it may 

 be paralysed as to taste while its general sensibility remains normal. In 

 the case of smell, on the other hand, paralysis of the nerves of general 

 sensibility, due to injury or diseases of the fifth, produces dry ness and 

 certain trophic changes which cause anosmia, although the fibres of the 

 olfactory nerve are not directly affected. 



While the lingual and glosso-pharyngeal nerves carry both nerves of 

 taste and of general sensibility, the very root of the tongue is supplied, 

 in part at any rate, by a nervous twig from the superior laryngeal 

 branch of the vagus, about whose function at present but little is 

 known. 



Taste sensations. Many of our so-called tastes are sensations 

 arising from stimulation of the nasal mucous membrane, but a certain 

 residue are quite odourless, and appeal alone to the mucous membrane 

 of the mouth. Salt, sweet, bitter, acid, are true tastes, and so probably 

 are metallic and astringent, though the nerves of tactile sensibility 

 have much to do with the feeling of astringency. 



Our power of tasting is much greater on some parts of the gustatory 

 mucous membrane than on others, being generally greater at the root 

 than at the tip of the tongue, and very slight over the middle line of 

 the tongue anteriorly. Our gustatory sensations exhibit, however, great 

 individual differences, more so perhaps than is the case with any other 

 sense organ, and although the above statement is the rule, there are 

 many exceptions ; for example, in the case of some persons, the front of 

 the tongue is extremely sensitive, while in others again it is hardly so 

 at all. 



1 F. Pope, Lancet, London, 1889, vol. ii. p. 458. 



2 Dixon, Trans. Roy. Irish Acad., Dublin, 1896. 



