gland, or nasal duct, is stimulated by the dryness, or
pained by the coldness of the air, or affected by any acrimonious dust or
vapour in the nostrils, it is excited into action together with the sack,
and the tears are disgorged upon the membrane, which lines the nostrils;
where they serve a second purpose to moisten, clean, and lubricate, the
organ of smell.

4. When the nasal duct of this gland is stimulated by any very acrid
material, as the powder of tobacco, or volatile spirits, it not only
disgorges the contents of its belly or receptacle (the lacrymal sack), and
absorbs hastily all the fluid, that is ready for it in the corner of the
eye; but by the association of its motions with those of the lacrymal
gland, it excites that also into increased action, and a large flow of
tears is poured into the eye.

5. This nasal duct is likewise excited into strong action by sensitive
ideas, as in grief, or joy, and then also by its associations with the
lacrymal gland it produces a great flow of tears without any external
stimulus; as is more fully explained in Sect. XVI. 8. on Instinct.

6. There are some, famous in the arts of exciting compassion, who are said
to have acquired a voluntary power of producing a flow of tears in the eye;
which, from what has been said in the section on Instinct above mentioned,
I should suspect, is performed by acquiring a voluntary power over the
action of this nasal duct.

7. There is another circumstance well worthy our attention, that when by
any accident this nasal duct is obstructed, the lacrymal sack, which is the
belly or receptacle of this gland, by slight pressure of the finger is
enabled to disgorge its contents again into the eye; perhaps the bile in
the same manner, when the biliary ducts are obstructed, is returned into
the blood by the vessels which secrete it?

8. A very important though minute occurrence must here be observed, that
though the lacrymal gland is only excited into action, when we weep at a
distressful tale, by its association with this nasal duct, as is more fully
explained in Sect. XVI. 8; yet the quantity of tears secreted at once is
more than the puncta lacrymalia can readily absorb; which shews _that the
motions occasioned by associations are frequently more energetic than the
original motions, by which they were occasioned_. Which we shall have
occasion to mention hereafter, to illustrate, why pains frequently exist in
a part distant from the cause o