1. _By stimulating the secreting vessels of the kidneys, by actual
contact._
_a_ The substance not undergoing any decomposition _in
transitu_.
It is easy to imagine that any substance which is capable of entering
the current of the circulation, and of stimulating the kidneys by a
direct application to their secerning vessels, may occasion a more
copious urinary discharge; in this manner the different saline[157]
preparations, _Potass_, _Soda_, _Nitrate of Potass_, _&c._ are brought
to the kidneys in the course of the circulation, and exciting the
vessels to an increased action, promote the secretion of a larger
proportion of watery fluid from the blood, in consequence of which, the
absorbents are indirectly stimulated in order to supply the deficiency,
and in this manner dropsical swellings are reduced.
Water, as a simple diluent, will promote the action of the kidneys, and
it is very judiciously remarked by Dr. Cullen, that by withholding the
use of fluids in dropsy, you will diminish the quantity of fluids
secreted, and allow the secretories of the kidneys to fall into a state
of inactivity and collapse. This is a sufficient answer to those who
defend the practice of enjoining an abstinence from all drinks in
dropsy.
b. _The diuretic remedy undergoing decomposition_ IN TRANSITU.
The digestive organs appear to possess the power of readily decomposing
all saline compounds into which _vegetable_ acids enter as ingredients,
and of eliminating their alkaline base, which, being in the course of
the circulation, carried to the kidneys, excites them into action, and
promotes the excretion of urine; and it is probably in this way that the
_Acetate_, _Citrate_, _Super-tartrate_, and other analogous combinations
of _Potass_ and _Soda_ prove diuretic: on the other hand, it is equally
evident that salts containing the _mineral_ acids are not under the
control of the decomposing powers of the _chylo-poietic_ organs, and
consequently do not undergo any changes _in transitu_, although some of
these salts, as I have just stated, especially the more soluble ones,
are absorbed entire, and prove diuretic. _Sulphate of Potass_, from its
insolubility, is not readily absorbed, and its composition will not
allow the developement of its base; we perceive therefore that it has
not any tendency to produce an influence upon the urinary secretion.
Certain vegetable bodies likewise appear to occasion diuresis by a
similar mode of operation, and it is worthy of notice that these
medicines generally contain a bitter principle, which is probably
separated by the analysing powers of the stomach; as exemplified in
_Scilla maritima_; _Colchicum autumnale_; _Lactuca Virosa_; _Gratiola
officinalis_; _Spartium Scoparium_ (Summitates); _Juniperus communis_;
_Copaifera Officinalis_ (Balsamum,) &c. The stimulant powers of a bitter
vegetable principle upon the _primæ viæ_, have already been fully
noticed under the consideration of TONICS, (_page 78_), and it is
reasonable to suppose, that an analogous principle, if introduced into
the circulation, may exert a corresponding impulse upon the organs with
which it comes into contact.
It particularly merits attention, that the diuretic operation of any
body that acts by being absorbed, is at once suspended if catharsis
follows its administration, whether in consequence of the largeness of
its dose, its increased solubility, or from the effect of its
combination with some purgative; for it is a law, _that the processes of
assimilation, and absorption from the duodenum, are arrested, or very
imperfectly performed during any alvine excitement_; the different
effects of the saline compounds of the alkalies with tartaric acid,
elucidate the truth of this law in a very striking manner—thus,
_Super-tartrate of Potass_, or Cream of Tartar, in well regulated doses,
acts, as we all know, upon the kidneys; the tartaric acid being, as I
suppose in this case, abstracted and assimilated by the digestive
process, and at the same time the alkaline base (Potass) eliminated, and
subsequently carried into the circulation; but if we increase the
solubility of the compound, by reducing it to the state of a neutral
tartrate (_soluble tartar_), or by combining it with _Boracic acid_, or
some body that has a similar effect; or what is equivalent to it, if we
so increase the dose[158] of the _cream of tartar_, that full catharsis
follows its administration, then diuresis will not ensue, since no
decomposition can take place under such circumstances, nor can it be
carried by absorption into the circulation. _Nitre_ and those salts
which are carried to the kidneys without previous decomposition _in
transitu_, are subject to the same law; for, if we combine them with
purgatives, their presence can no longer be recognised in the urine, as
I have ascertained by experiment. _Oil of Turpentine_ in doses of two
fluid-drachms, may so excite the urinary organs as to produce even
bloody urine; whereas a fluid-ounce will scarcely occasion any apparent
influence upon those functions, because the increased dose acts upon the
bowels, and consequently prevents its passage into the circulation.
_Sulphate of Magnesia_ does not readily produce any diuresis, because it
operates upon the bowels, but the experiments of Vitet and Bracy Clarke
have shewn, that if this saline compound be administered to the horse
whose bowels are not easily affected by purgatives, it acts powerfully
upon the kidneys;[159] and I will take occasion in this place to observe
that, on account of the inirritability of the bowels of the horse,
diuretic medicines are more certain in their operation, than in the
human subject; a fact which, in itself, shews the importance of
attending to the state of the bowels, during a course of those diuretics
which require to be absorbed before they can produce their specific
effects.
Equally necessary is it to attend to the state of the vessels of the
skin, for if during the administration of a Diuretic, these vessels be
excited by external warmth, its action may be diverted from the urinary
organs to the exhalants on the surface, and occasion diaphoresis; but if
the surface of the body be kept cool, this diversion will not occur: so
greatly indeed does cooling the surface determine to the kidneys, that
the usual diaphoretic medicines may, by an attention to this
circumstance, be converted into powerful diuretics.
C. II. MEDICINES WHICH ACT _Primarily_ ON THE ABSORBENTS, AND
_Secondarily_ ON THE KIDNEYS.
It has been shewn, in the former division, that by increasing the action
of the kidneys, we diminish the quantity of water in the blood, and
consequently occasion an extraordinary action of the absorbents to
supply the deficiency, whence dropsical accumulations disappear; it
remains to be stated that an operation, which may be considered the
converse of the one just described, is not unfrequently established: the
absorbent vessels, in this case, are first roused to extraordinary
action, and the blood therefore becomes surcharged with serous matter,
in consequence of which the kidneys are stimulated, and it is eliminated
through the urinary passages: so that in the former case the absorbent
may be said to be called into action by the kidneys, while in the
latter, the kidneys are obviously subservient to the increased energy of
the absorbent system. The preparations of Mercury are perhaps the only
medicinal bodies which we can strictly consider as specific stimulants
to the absorbent system; and of their power in directly acting upon
these organs there are such ample proofs, that it is unnecessary to
adduce any additional evidence upon the subject. In instances of
increased absorption from the agency of other medicines, the effect must
be considered as rather arising from their _secondary_ than primary
operations; the most important of which will constitute objects of
inquiry in the succeeding divisions of the subject.
C. III. MEDICINES WHICH ACT _Primarily_ ON THE STOMACH OR SYSTEM, AND
_Secondarily_ ON THE URINARY ORGANS.
A Diuretic effect is very frequently occasioned by substances which act
on the stomach and _primæ viæ_, producing a peculiar state of these
organs, which _sympathetically_ affects the whole body, and more
particularly the absorbent system, and the vessels concerned in the
secretion of urine from the blood. As this primary influence upon the
stomach, and the effects to which it gives rise in remote parts, are
very different in their character, according to the nature of the remedy
employed, and the state of the system at the time of its administration;
the present attempt to investigate and generalize these relations, and
to adopt them as the basis of a classification, may ultimately lead the
practitioner to some distinctions of practical utility.