Substances which, when applied to the human body, corrugate and condense
its fibres, and at the same time, exert a tonic influence through the
medium of its living principle.
Astringency in any substance may be at once recognised by the organs of
taste; its power in corrugating the papillæ of the tongue, and in
imparting a sensation of harshness and roughness to the palate, being
too peculiar to be mistaken; this is a fortunate circumstance, for there
does not exist any one chemical test by which we can invariably detect
the property of astringency, since it is found to reside in many
different classes of substances: thus, acids, especially the stronger
mineral ones, are powerfully astringent; so are many of the metallic
salts, as those of iron, zinc, copper, and lead; and some of the earths,
when combined with acids, of which alum is a striking example. The
vegetable kingdom, however, furnishes the greater number of astringent
remedies; and chemistry has shewn that this property uniformly depends
upon a peculiar proximate principle, characterized by its power of
forming an insoluble compound with animal gelatine; to this principle
the name of TANNIN has been given. As tannin generally exists in union
with gallic acid, and as the latter body is known by its property of
striking an inky blackness with the salts of iron, solutions of this
metal were long, but erroneously, regarded as the proper test of
vegetable astringency; the fallacy of this is at once shewn by the
habitudes of Catechu, one of the strongest of our astringents, but
which, nevertheless, will not yield the smallest degree of blackness to
the solutions of iron, because it contains only tannin, the true
principle of astringency, without a trace of its usual associate the
gallic acid. From the power which these substances possess of
astringing, and condensing the animal solids, their medicinal properties
are supposed to arise, and we may perhaps, in this instance, admit such
a mechanical explanation; but astringents possess also some power over
the living principle of the matter which they astringe, for they are
capable of acting as permanent stimulants, of curing intermitting fever,
and of obviating states of general debility. Astringents would seem to
moderate the morbidly increased secretions of distant parts, and to
restrain hemorrhage, by their corrugating influence upon the _primæ
viæ_,[148] which is extended by sympathetic action to the vascular
fibre; it is not difficult for any person to conceive the possibility of
such a sympathy, who has ever experienced the thrilling and singular
feeling which is produced over the whole body, by the _acerb_ taste of
the sloe-juice. As however the primary operation of these bodies, by
their actual contact with the animal fibre, must be much more powerful
than that which can result from the mere sympathy of parts, we find that
the efficacy of astringents is principally displayed in the cure of
diarrhœa, or serous evacuations from the intestinal canal; their
operation, in checking profuse fluor albus, gleet, and the inordinate
secretions of other distant organs, is much less striking and
unequivocal, and it is a question whether in many of such cases the
benefit arising from their use may not depend upon their tonic powers.
As the morbid excess of different evacuations may arise from various and
opposite states of the living system, so may the individuals of the
other classes become astringents; and we are bound to admit upon this,
as we have on other occasions, the existence of _absolute_ and
_relative_ remedies.
Narcotics, at the head of which stands opium, will frequently assume the
character of astringents, by diminishing the irritability upon which
increased discharges depend. In Diarrhœa, an astringent, properly so
called, diminishes the flow of those acrid fluids into the intestines,
by which their peristaltic motions are præternaturally increased, and it
consequently represses the diarrhœa; a narcotic, under similar
circumstances, might not repress the flow of the acrid matter to which I
have alluded, but it would render the bowels less susceptible to its
stimulus, and would therefore produce the same apparent alleviation,
although by a very different mode of operation. There is yet a third
species of remedy, which may operate in restraining a diarrhœa of this
description; not by stopping the flow of acrid matter, nor by
diminishing the irritability of the intestinal organs, as in the
instances above recited, but, simply, by _acting chemically_ upon the
offending matter, so as to disarm it of its acrid qualities; such, for
instance, is the nature of _absorbent_ and _testaceous_ medicines. In
the cure of hemorrhage, if it be _active_, that is to say, connected
with a state of strong tonic contractility of the blood-vessels, a very
different remedy will be required as an astringent, than in cases of
_passive_ hemorrhage, in which the vascular fibres are in a state of
relaxation or collapse. Sir Gilbert Blane has offered some valuable
remarks upon this subject, with a view to settle the difference of
opinion which has arisen respecting the treatment of flooding after
child-birth. (_Medical Logic_, _Edit._ 2d. p. 100.)
Astringents are capable of being exclusively used as local applications,
and when they are so employed for the purpose of stopping hemorrhage,
they are termed STYPTICS.[149] With respect to these latter agents it
must be confessed, that great popular error still exists, much of which
has evidently arisen from deductions drawn from the effects of such
remedies upon inferior animals; thus have several substances gained the
reputation of _Styptics_, from the result which may have followed their
application to the wounded and bleeding vessels in the extremities of
the horse and ass; whereas the fact is, that the blood-vessels of these
animals possess an inherent power of contraction which does not exist in
those of man, and to which alone the cessation of the hemorrhage,
fallaciously attributed to the _Styptic_, is to be wholly attributed. In
many cases an application may owe its styptic qualities to its power of
coagulating the blood around the orifice of the wound; in this way the
contact of heated metal will sometimes arrest the flow of blood from a
cut surface.