1. AS DERIVATIVES, i. e. _by producing a derivation of the circulation
from the inflamed and engorged vessels of the neighbouring organs
to the blistered surface_. This mode of operation was long
overlooked by the physicians, who ascribed all the beneficial
effects of a blister to the evacuation which it produced, while
the humoral pathologist, moreover, considered the matter so
discharged to be of a morbific nature. That such agents owe their
salutary tendency to causes independent of their powers as
evacuants, is at once rendered evident by the relief which they
afford, when used only as Rubefacients.
2. AS EVACUANTS—_by occasioning an effusion of Fluids_. In this case
the vesicated part may be considered in the light of a new
excretory organ, the formation of which requires the establishment
of a new current or determination of blood; so long as the
discharge continues, so long will there be an especial demand of
blood in the blistered part, and a consequent derivation of the
circulation from the inflamed and engorged vessels of the
neighbouring organs.[174] The nature of the fluid effused is at
first serous, but after some time it becomes purulent, and this
stage of its operation must be considered as, by far, the most
beneficial; hence the great advantages derived from a “_perpetual
blister_.”
3. AS GENERAL STIMULANTS, _by raising the vigour of the
circulation_.[175] That Blisters have such a tendency there exist
too many proofs to allow us to doubt. Hence in fevers they
frequently prove valuable auxiliaries, but since the application
of any stimulus, in such diseases, must be regulated by the degree
of excitement, it is evident that they can only be made with
success in particular stages; this simple fact will at once
explain the cause of that want of unanimity in Physicians with
respect to the value of blisters in febrile diseases. Rush
considered that there was one particular period, in the course of
a continued fever, intermediate between its stage of high
excitement and the appearance of a collapse, in which blisters
will generally produce unequivocal good effects, and to this he
gave the name of the _Blistering point_.
4. AS ANTISPASMODICS.—_Relieving pain through the medium of Contiguous
Sympathy._ This effect would frequently appear to be independent
of the operations above enumerated; a similar principle seems to
exist with regard to the _pain_ excited by blisters, which may
also be applied to the explanation of the advantages derived from
them in several diseases. It has long been remarked that, by
exciting one pain we may often relieve another, and hence blisters
afford relief in tooth-ache, and other painful affections.
Epilepsy and Hysteria, arising from irritation, have been removed
by such applications, apparently from their exciting powers.
It remains for us to make a few observations upon the abuse of these
remedies, for, notwithstanding the popular adage that “_Blisters are
always safe things_,” that “_if they do no good, they can do no harm_,”
they will be found, like all other potent applications, capable of
producing much mischief when directed by unskilful hands. In stages of
high vascular excitement in the pulmonary organs, blisters have
increased the irritation they were designed to allay, and in some cases
have promoted a tendency to effusion; in the treatment of acute
Hydrocephalus the common practice of blistering the head appears very
questionable, and has too often, I am well persuaded, accelerated the
fatal termination, by increasing the disposition to serous effusion.
ISSUES (_Fonticuli_) and SETONS (_Setacea_[176]).
The effects of these processes bear a strong analogy to those which are
produced by Vesicatories; they are, however, more permanent, and are on
that account better adapted to the relief of those chronic affections
which would seem to require a remedy of long continued influence. In
pulmonary affections, for instance, a seton in the side is frequently
attended with very considerable benefit. The popular belief in humoral
pathology, which continues to influence the mass of mankind, has perhaps
assigned to these remedies a greater share of credit than that to which
they are really entitled, but it must still be acknowledged that when an
ulcer having existed a great length of time, is healed or _dried up_, or
any constitutional discharge is suddenly checked, the health may become
affected. In such cases the establishing a discharge by means of an
Issue is undoubtedly a safe, and often a beneficial operation.