Hunger is most probably perceived by those numerous ramifications of nerves
that are seen about the upper opening of the stomach; and thirst by the
nerves about the fauces, and the top of the gula. The ideas of these senses
are few in the generality of mankind, but are more numerous in those, who
by disease, or indulgence, desire particular kinds of foods or liquids.

A sense of heat has already been spoken of, which may with propriety be
called an appetite, as we painfully desire it, when it is deficient in
quantity.

The sense of extension may be ranked amongst these appetites, since the
deficiency of its object gives disagreeable sensation; when this happens in
the arterial system, it is called faintness, and seems to bear some analogy
to hunger and to cold; which like it are attended with emptiness of a part
of the vascular system.

The sense of want of fresh air has not been attended to, but is as distinct
as the others, and the first perhaps that we experience after our nativity;
from the want of the object of this sense many diseases are produced, as
the jail-fever, plague, and other epidemic maladies. Animal love is another
appetite, which occurs later in life, and the females of lactiferous
animals have another natural inlet of pleasure or pain from the suckling
their offspring. The want of which either owing to the death of their
progeny, or to the fashion of their country, has been fatal to many of the
sex. The males have also pectoral glands, which are frequently turgid with
a thin milk at their nativity, and are furnished with nipples, which erect
on titillation like those of the female; but which seem now to be of no
further use, owing perhaps to some change which these animals have
undergone in the gradual progression of the formation of the earth, and of
all that it inhabit.