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34 — DENTAL MEDICINE.

established should be, in a great measure, regulated by the char-
acter of the disease which it is intended to relieve ; a rubefacient
being indicated in irritation of mucous membrane, a seton or
issue when the disease is of a suppurative character, and a vesi-
cant in inflammation of serous membranes.
Setons and Issues are employed to produce permanent counter-
irritant effects. A seton consists of a skein of silk or a piece of
tape or other substance passed through the integument by means
of a seton-needle, and allowed to remain, so that a discharge is
maintained. A simple seton in the case of an incision into an
alveolar abscess, is composed of a single or a double strand of
floss silk introduced into the wound made by the lancet, or into
the orifice of a fistulous opening, after the pus of the abscess has
been evacuated, to maintain a free exit for the pus which may be
fine silver wire is also employed
secreted after the first operation ;
for the same purpose. An issue is generally some irritating sub-
stance, such as caustic potassa, or a small pea, or piece of orris
root introduced in order to maintain a discharge.
Blood-letting is performed for the purpose of lessening vascular
excitement, reducing inflammatory action, relieving congestive
pain and spasm, promoting absorption, relaxing the muscles and
arresting hemorrhage. It is divided into ^^n^ra/ and /or«/, general
blood-letting consisting of venesection or phlebotomy, the median
cephalic or basilic veins of the arm, and occasionally the external
jugular and other veins, being the ones selected from which to
draw the blood. But it should be resorted to with caution, as it
is a powerful and exhausting agent.
Local Blood-letting is chiefly employed for the relief of local
inflammations and congestions, and is accomplished by means of
leeches, cups, and scarifications.
The leech hirudo— is commonly employed as an agent for
local blood-letting, and is preferable to " cupping " in many local
and chronic forms of inflammation also in
; infantile aff^ections
which require such an operation, when the American leech is
used, and it makes a smaller incision than the European leech,
and draws less blood. A leech is supposed to draw on an average,
about a drachm and a half to two drachms of blood before it is