Thirst is opposite to hunger, and is a general attendant upon fevers. It follows also, in an evident manner, from all considerable degrees of heat in the fauces. The nerves therefore of the mouth, fauces, œsophagus, and stomach, are, during thirst, preoccupied by disagreeable vibrations, of the inflammatory kind, as above observed. And as the pleasures of taste may be said to resemble those of titillation, so thirst seems allied to itching.
It is agreeable to this account of thirst, that liquors actually cold afford immediate relief; also that warm diluents, which soften the parts, and wash off acrimonious particles, do it after some time. As the cause of thirst is of a permanent nature in fevers, it must return again and again, till the fever be removed. Gentle acids yield a pleasure in thirst, which seems to correspond to that which scratching excites, in parts that itch previously.
Some of these changes are,
That sweets generally grow less and less agreeable, and sometimes even disagreeable, or nauseous at last.
That astringent, acid, and spirituous liquids, which displease at first, afterwards become highly grateful.
That even bitters and acrids first lose their offensive qualities, and after a sufficient repetition give a relish to our aliment.
And that many particular foods and medicines become either extremely pleasant or disgusting, from associations, with fashion, joy, hope of advantage, hunger, the pleasures of cheerful conversation, &c. or with sickness, vomitings, gripings, fear, sorrow, &c.
Now, in order to account for these changes, we may consider the following things.
First, That the organs become less and less sensible by age, from the growing callosity and rigidity of all the parts of animal bodies. The pleasant savours may therefore be expected to become less pleasant, and the moderately disagreeable ones to fall down within the limits of pleasure, upon this account.
Secondly, The disposition to vibrations in the organ and corresponding part of the brain must also receive some alteration by the frequent repetition of impressions. For though this returns, at a proper distance from each meal, to its former state, within an indefinite distance, as one may say, yet some difference there probably is, upon the whole, which in a sufficient length of time amounts to a perceptible one. However, we must also suppose on the other hand, that the make of the nervous system sets some bounds to this gradual alteration in the disposition to vibrate;
