140 ^ ON THE SENSES. 



ailments above spoken of, it is safe to make of the more or less thickened 

 and furred envelope of the tongue a scape-goat on which we may cast, in the 

 first instance, the blame of the defective or vitiated sensibility to objects of taste. 

 We may suppose, moreover, that in catarrh, exudations from the mucous mem- 

 brane clog its tissues, and thus impair the irritability of the extremities of the 

 nerves, or in some way embarrass the yet unexplained action of sapid substances 

 on those nerves. But such an explanation or supposition does not carry us far. 

 What really gives rise to the incidental perversion of the sensations in question, 

 can scarcely be the subject of even plausible conjecture, so long as we know not 

 the nature of that excitation which different substances produce in the nerves, so 

 long as we are ignorant how a sweet or bitter taste comes to be generated. That 

 after a sour substance, a sweet one tastes doubly sweet, the reader may be dis- 

 posed to explain by the effect of contrast. This explanation we shall not in general 

 contest, but most earnestly protest against the commonly received notion that a 

 sweet and a sour taste are opposites of one another. Between two sensations of 

 different quality, whether they belong to the same or different spheres of sense, 

 there exists no tertium comparationis, and so nothing which will authorize its to 

 regard them as opposites. It is just as unreasonable to consider a sour taste the 

 opposite of a sweet one, as to attribute opposition to the sensations of a red and 

 green color, or a shrill and a deep tone, as if the one stood in diametric relation 

 to the other as a north or south pole. Could the two sensations be even assigned 

 to external causes essentially antithetical, weshould not be justified in transferring 

 this character to the sensations themselves. These are incapable of all descrip- 

 tion ; they admit not of measurement or division, and can as little be brought into 

 the relationship of opposition as of numerical value. 



We must not pass without at least a brief notice the qfter-iaste or tang, 

 although in truth its nature and causes are as obscure as those of tlje proper 

 sensations of taste. This after -taste exists in a twofold manner. With some 

 substances, the original sensation which they create continues of the same un-* 

 changed quality for a long time after they have been swallowed and been dis- 

 placed on the tongue by other substances. With others, again, after the retreat 

 of the sensation originally excited, another supervenes of quite different quality. 

 Thus there are bitter substances whose taste is not to be got rid of, as it is 

 commonly expressed ; and on the other hand, sweet substances which create a bit- 

 ter after-taste. When this after-taste is of like quality with the original sensa- 

 tion, it is not easy to decide whether it is of a purely objective nature, or in 

 part, at least, subjective; that is to say, whether the action of the substance 

 which generates the primary sensation itself persists as long as the after-taste 

 endures, or whether, for some reason, the excitation of the nerves, or the activity 

 of that central apparatus which operates directly on the sensorium and deter- 

 mines the nature of the sensation, continues even after the disappearance of the 

 substance from the environs of the nerve extremities. Of the last there is no 

 existing proof; the objective nature of the after-taste is, on the other hand, 

 readily conceivable and capable of explanation in different ways. It is very 

 possible that certain substances, having once penetrated into the mucous mem- 

 brane, cling more pertinaciously than others to its structural tissues, without 

 being removed, by absorption or chemical decomposition, i'rom the vicinity of 

 the nerve extremities on which, therefore, they continue to operate. The prob- 

 abilities in favor of this origin of the after-taste are greater, the smaller the 

 quantities of the substance necessary to produce the sensation. In the second 

 place, it may well be supposed that there are substances capable of affecting 

 the nerves of taste through the blood; indeed, it is certain that many ''sub- 

 jective" sensations of taste, which, from time to time, present themselves with- 

 out any external cause, are in reality objective in the sense that substancea 

 conveyed from the bowels into the blood have been in such cases carried by 

 the latter to the mucous membrane of the tongue, and thus reach the sensitive 



