imulus, one of which may be compared to a pleasurable
sensation or desire inducing the vessel to seize, and, as it were, to
swallow the particles thus selected from the blood; as is done by the
mouths of the various glands, veins, and other absorbents, which may be
called glandular appetency. The other kind of stimulus may be compared to
disagreeable sensation, or aversion, as when the heart has received the
blood, and is stimulated by it to push it forwards into the arteries; the
same again stimulates the arteries to contract, and carry forwards the
blood to their extremities, the glands and capillaries. Thus the mesenteric
veins absorb the blood from the intestines by glandular appetency, and
carry it forward to the vena portarum; which acting as an artery contracts
itself by disagreeable stimulus, and pushes it to its ramified extremities,
the various glands, which constitute the liver.

It seems probable, that at the beginning of the formation of these vessels
in the embryon, an agreeable sensation was in reality felt by the glands
during secretion, as is now felt in the act of swallowing palatable food;
and that a disagreeable sensation was originally felt by the heart from the
distention occasioned by the blood, or by its chemical stimulus; but that
by habit these are all become irritative motions; that is, such motions as
do not affect the whole system, except when the vessels are diseased by
inflammation.

* * * * *

SECT. XXIV.

OF THE SECRETIONS OF SALIVA, AND OF TEARS, AND OF THE LACRYMAL SACK.

I. _Secretion of saliva increased by mercury in the blood._ 1. _By the
food in the mouth. Dryness of the mouth not from a deficiency of
saliva._ 2. _By Sensitive ideas._ 3. _By volition._ 4. _By distasteful
substances. It is secreted in a dilute and saline state. It then
becomes more viscid._ 5. _By ideas of distasteful substances._ 6. _By
nausea._ 7. _By aversion._ 8. _By catenation with stimulating
substances in the ear._ II. 1. _Secretion of tears less in sleep. From
stimulation of their excretory duct._ 2. _Lacrymal sack is a gland._ 3.
_Its uses._ 4. _Tears are secreted, when the nasal duct is stimulated._
5. _Or when it is excited by sensation._ 6. _Or by volition._ 7. _The
lacrymal sack can regurgitate its contents into the eye._ 8. _More
tears are secreted by association with the irritation of the nasal duct
of the lacrymal