eat along
with the light must be concentrated upon it. Nor does warm water or warm
oil poured into the ear give pain to the organ of hearing; and hence as
these organs of sense do not perceive small excesses or deficiences of
heat; and as heat has no greater analogy to the solidity or to the figures
of bodies, than it has to their colours or vibrations; there seems no
sufficient reason for our ascribing the perception of heat and cold to the
sense of touch; to which it has generally been attributed, either because
it is diffused beneath the whole skin like the sense of touch, or owing to
the inaccuracy of our observations, or the defect of our languages.

There is another circumstance would induce us to believe, that the
perceptions of heat and cold do not belong to the organ of touch; since the
teeth, which are the least adapted for the perceptions of solidity or
figure, are the most sensible to heat or cold; whence we are forewarned
from swallowing those materials, whose degree of coldness or of heat would
injure our stomachs.

The following is an extract from a letter of Dr. R.W. Darwin, of
Shrewsbury, when he was a student at Edinburgh. "I made an experiment
yesterday in our hospital, which much favours your opinion, that the
sensation of heat and of touch depend on different sets of nerves. A man
who had lately recovered from a fever, and was still weak, was seized with
violent cramps in his legs and feet; which were removed by opiates, except
that one of his feet remained insensible. Mr. Ewart pricked him with a pin
in five or six places, and the patient declared he did not feel it in the
least, nor was he sensible of a very smart pinch. I then held a red-hot
poker at some distance, and brought it gradually nearer till it came within
three inches, when he asserted that he felt it quite distinctly. I suppose
some violent irritation on the nerves of touch had caused the cramps, and
had left them paralytic; while the nerves of heat, having suffered no
increased stimulus, retained their irritability."

Add to this, that the lungs, though easily stimulated into inflammation,
are not sensible to heat. See Class. III. 1. 1. 10.

VII. _Of the Sense of Extension._

The organ of touch is properly the sense of pressure, but the muscular
fibres themselves constitute the organ of sense, that feels extension. The
sense of pressure is always attended with the ideas of the figure and
solidity of the object, neither of which a