THE SENSE OF TASTE. 135 



illustrateourinc.apacity of arriving by physical or chemical analysis at any decisive 

 results on this subject, we shall cite one or two examples. Sugar may serve as the 

 representative of sweet substances ; we know that it is composed of twelve atoms 

 of carbon, hydrogen, and oxygen, and how it is affected by all chemical agents ; we 

 know, also, its physical properties, such as solubility in equivalent proportions, 

 requiring here no further discrimination. Plow is it now that this substance, on 

 its contact, in solution, with the mucous membrane of the tongue, produces an 

 excitation, whether chemical, thermal, electrical, or mechanical, which gives rise, 

 by its action on the ends of the nerves, to that specific modification of the 

 nervous current to which an appropriate sensation of taste instantly icsponds 

 in the brain ? We may well suppose that the sugar does not take effect on the 

 nerves' hidden under the epithelium until imbibed by the latter, and thus brought 

 into proximity with their extremities ; but even if it penetrated into immediate 

 contact with them, in what respect is the solution of the question advanced ? If 

 we bring dissolved sugar into contact with the nerves of the muscles not a fibre 

 stirs; a sufficient proof that sugar is in itself no "adequate" irritant of the 

 nerves. AVhat, then, qualifies the sugar for excitation of the nerves of taste ? 

 The obvious answer is, the presence of some special arrangement at the ends of 

 these nerves. But of Avhat sort is it 1 How and by what means does the sugar 

 operate upon it ? How, through those means, is a nervous excitation produced ? 

 This is the riddle to be solved, but it offers at present so much resistance to the 

 sharpest powers of divination that we decline any attempt to meddle with it. 

 The matter becomes still more inexplicable when we consider that, in the first 

 place, sugar is not the only substance that tastes sweet, but that a multitude of 

 other substances, in no respect resembling it either physically or chemically, pro- 

 duces the same sensation in the organs of taste ; and, in the second place, that 

 slill other substances, which are most nearly related to sugar, having even the 

 same composition, generate either no sensation at all or produce oneof a wholly 

 different character. What, for instance, have sugar and a combination of lead 

 with acetic acid in common that both should taste sweet 1 What sulphate of 

 magnesia and quinine that both should taste bitter 1 What distinguishes sugar 

 from starch that the former should produce a sweet taste, the latter no taste at 

 all ? No answer can be given, and we should but freight our page with useless 

 verbiage did we give a list of all the substances having properties thus compara- 

 ble. As regards other senses, that of sight, for example, we are in abetter posi- 

 tion. True, that even here we do not know in what consists the operation of 

 light on the extremities of the optic ilerve, whence result their excitation and 

 mediately the sensation of light. But Ave do know, in the first place, that at 

 those extremities there exists a wonderfully complicated apparatus in which Ave 

 can recognize the intermediary between light and nerve, through whose agency 

 the former becomes an adequate irritant of the latter ; and we know, in the second 

 place, by what momentum of the outAvard agent, light, the quality of the sen- 

 sation is determined ; it is practicable to state mathematically the degree of mo- 

 mentum adapted to each sensation. The undulatory movements of the luminous 

 ether produce the excitation of the nerves of sight ; the velocity of those movements 

 qualifies the sensation so as to produce the colors which we see. A definite 

 amount of this velocity is universally necessary for the production of the excita- 

 tion ; Ave are able to specify the smallest number of oscillations in a second of 

 time AA'hich cannot be diminished, the largest number which cannot be increased, 

 without producing in either case an extinction of the effect of the Avaves of light 

 on the visual nerves; we are able, also, to state Aviih entire certainty the unal- 

 terable number of oscillations respectively pertaining to the perceptions to whicli 

 Ave give such names as red, green, yellow, or blue. Bo it is, also,Avith rcg-ard to 

 the sense of hearing ; Avhat has just been said of the waves of light and their 

 relations to the organs of vision and impressions of color may be translerred, 

 with only verbal ':»hanges. to the Avaves of sound which produce the excitation 



