148 SENSE OF TASTE. 



When the notion was once entertained, that a sapid principle is an 

 integrant molecule, sapidity was attempted to be explained by its shape. 

 It was said, for instance, that if the savour be sweet, the molecule must 

 be round; if sharp, angular; and so forth. Sugar was said to possess a 

 spherical, acids, a pointed, or angular molecule. We know, however, 

 that substances which resemble each other in the primitive shape of their 

 crystal, impress the organ of taste differently ; and that solution, which 

 must destroy most if not all the influence from shape, induces no 

 change in the savour. 



Others have referred sapidity to a kind of chemical action between 

 the molecules, and the nervous fluid. This view has been suggested by 

 the fact, that, as a general principle, sapid, like chemical bodies, act 

 only when in a state of solution ; that the same savours usually belong 

 to bodies possessed of similar chemical properties, as is exemplified by 

 the sulphates and nitrates; and that, in the action of acids on the tongue 

 and mouth, we witness a state of whiteness and constriction, indicative 

 of a first degree of combination. All these circumstances, however, 

 admit of another explanation. There are unquestionably many sub- 

 stances, which do combine chemically, not with a nervous fluid, of 

 whose existence we know nothing, but with the mucus of the mouth ; 

 and the sapidity resulting from such combination is appreciated by the 

 nerves of taste ; but there are many bodies, which are eminently sapid, 

 and yet afford us instances of very feeble powers of chemical com- 

 bination ; nay, in numerous cases, we have not the least evidence that 

 such powers exist. Vegetable infusions or solutions are strong ex- 

 amples of the kind, of which syrup may be taken as the most fami- 

 liar. The effect of solution is easily intelligible ; the particles of the 

 sapid body are in this way separated, and come successively into 

 contact with the gustatory organ ; but there is some reason to believe, 

 that solution is not always requisite to give sapidity. Metals have 

 generally a peculiar taste, which has been denominated metallic ; and 

 this, even if the surface be carefully rubbed, so as to free it from oxide, 

 which is more or less soluble. Birds, too, whose organs of taste are 

 as dry as the corn they select from a mass of equally arid substances, 

 are probably able to appreciate savours. The taste produced by touch- 

 ing the wires of a galvanic pile with the tongue has been offered as 

 another instance of sapidity exhibited by dry bodies. This is, more 

 probably, the effect of the chemical action on the fluids covering the 

 mucous membrane of the tongue, which always follows such contact. 

 Such chemical change must, however, be confined to these fluids; and, 

 when once produced, the nerve of taste is impressed by the savour de- 

 veloped in the same manner as it is in cases of morbid alterations of 

 the secretion of the mucous membrane. In both cases, a body pos- 

 sessing considerable and peculiar sapidity may fail to impress the 

 nerves altogether, or may do so inaccurately. The notion of any che- 

 mical combination with the nervous fluid must of course be discarded, 

 as there is not the slightest evidence in favour of the hypothesis ; yet 

 the epithet chemical was once applied to this sense on the strength of 

 it; in opposition to the senses of touch, vision, and audition, which 

 were called mechanical, and supposed to be produced by vibrations of 

 the nerves of those senses. 



