y be known by applying one's hand
upon the heart as above; but the principal cause I suppose to consist in
the diminution of sensorial power. When a muscle contains, or is supplied
with but little sensorial power, its contraction soon ceases, and in
consequence may soon recur, as is seen in the trembling hands of people
weakened by age or by drunkenness. See Sect. XII. 1. 4. XII. 3. 4.

It may nevertheless frequently happen, that both the deficiency of
stimulus, as where the quantity of blood is lessened (as described in No.
4. of this section), and the deficiency of sensorial power, as in those of
the temperament of irritability, described in Sect. XXXI. occur at the same
time; which will thus add to the quickness of the pulse and to the danger
of the disease.

III. 1. A certain degree of heat is necessary to muscular motion, and is,
in consequence, essential to life. This is observed in those animals and
insects which pass the cold season in a torpid state, and which revive on
being warmed by the fire. This necessary stimulus of heat has two sources;
one from the fluid atmosphere of heat, in which all things are immersed,
and the other from the internal combinations of the particles, which form
the various fluids, which are produced in the extensive systems of the
glands. When either the external heat, which surrounds us, or the internal
production of it, becomes lessened to a certain degree, the pain of cold is
perceived.

This pain of cold is experienced most sensibly by our teeth, when ice is
held in the mouth; or by our whole system after having been previously
accustomed to much warmth. It is probable, that this pain does not arise
from the mechanical or chemical effects of a deficiency of heat; but that,
like the organs of sense by which we perceive hunger and thirst, this sense
of heat suffers pain, when the stimulus of its object is wanting to excite
the irritative motions of the organ; that is, when the sensorial power
becomes too much accumulated in the quiescent fibres. See Sect. XII. 5. 3.
For as the peristaltic motions of the stomach are lessened, when the pain
of hunger is great, so the action of the cutaneous capillaries are lessened
during the pain of cold; as appears by the paleness of the skin, as
explained in Sect. XIV. 6. on the production of ideas.

The pain in the small of the back and forehead in the cold fits of the
ague, in nervous hemicrania, and in hysteric paroxysms, when all the
irritat