ng System._

These are always attended with increase of partial or of general heat; for
the secreted fluids are not simply separated from the blood, but are new
combinations; as they did not previously exist as such in the blood
vessels. But all new combinations give out heat chemically; hence the
origin of animal heat, which is always increased in proportion to the
secretion of the part affected, or to the general quantity of the
secretions. Nevertheless there is reason to believe, that as we have a
sense purposely to distinguish the presence of greater or less quantities
of heat, as mentioned in Sect. XIV. 6. so we may have certain minute glands
for the secretion of this fluid, as the brain is believed to secrete the
sensorial power, which would more easily account for the instantaneous
production of the blush of shame, and of anger. This subject deserves
further investigation.

SPECIES.

1. _Calor febrilis._ The heat in fevers arises from the increase of some
secretion, either of the natural fluids, as in irritative fevers; or of new
fluids, as in infectious fevers; or of new vessels, as in inflammatory
fevers. The pain of heat is a consequence of the increased extension or
contraction of the fibres exposed to so great a stimulus. See CLASS I. 1.
5. 6.

2. _Rubor febrilis._ Febrile redness. When the cold fit of fever
terminates, and the pulsations of the heart and arteries become strong as
well as quick from the increase of their irritability after their late
quiescence, the blood is impelled forwards into the fine extremities of the
arteries, and the anastomozing capillaries, quicker than the extremities of
the veins can absorb and return it to the heart. Hence the pulse at the
wrist becomes full, as well as quick and strong, and the skin glows with
arterial blood, and the veins become empty and less visible.

In elderly people the force of the heart and arteries becomes less, while
the absorbent power of the veins remains the same; whence the capillary
vessels part with the blood, as soon as it is received, and the skin in
consequence becomes paler; it is also probable, that in more advanced life
some of the finer branches of the arteries coalesce, and become impervious,
and thus add to the opacity of the skin.

3. _Sudor calidus._ Warm sweat may be divided into four varieties,
according to their remote causes. _First_, the perspirable matter is
secreted in as great quantity during the hot fit of fever, as to