Substances which increase the salivary[166] discharge. This class
comprehends two orders of medicines, viz.
1st. Those which increase the salivary excretion by _external_
application to the secreting vessels, by mastication, as the following
acrimonious and pungent substances, _Anthemis Pyrethrum_; _Colchearia
Armoracia_; _Daphne Mezereum_; _Nicotiana Tabacum_, &c.
2nd. Those whose _internal_ exhibition affects these organs through the
medium of the circulation, of which _Mercury_ is the only true example;
for all the preparations of this metal, when administered in certain
quantities, produce salivation.
The acrid Sialogogues, or Masticatories, by stimulating the excretory
ducts, and increasing the secretion of saliva, sometimes relieve the
pain of tooth-ache, and are commonly resorted to for that purpose; they
are besides supposed capable of relieving other congestions, or
inflammatory dispositions, in remote parts of the head, by the
derivation they occasion from the neighbouring vessels, especially the
branches of the external carotid.
Mercury, in its metallic state,[167] is perfectly inert, and does not
exert any influence whatever upon the living body: this fact is
sufficient, if any serious refutation were necessary, to overturn the
theory which attributes its sialogogue property to the gravity of its
particles, by which “it is disposed to retain the _direct line_ in which
it is propelled from the heart, and is therefore more certainly
determined to the vessels of the head.” It has been also supposed to act
by diminishing the _lentor_ of the blood, and disposing it to pass more
easily into the salivary glands, so as to increase their secretion:
equally gratuitous and improbable are the chemical hypothesis which have
been offered to explain this curious and singular property; Dr. Cullen
endeavoured to solve the problem, by supposing that “_Mercury has a
particular disposition to unite with ammoniacal salts, and that such
salts are disposed to pass off by the salivary glands more copiously
than by any other excretion_.” Dr. Murray, however, very justly remarks
that mercury has not any peculiar tendency of this kind; and that if it
had, these salts are not more abundant in the saliva, than in some other
secretions. Dr. Murray then proceeds to submit a theory which he
considers better calculated to explain the phenomenon; he observes, that
the urine appears more peculiarly designed to convey matter which has
been received into the circulating mass, but which is still
excrementitious, from the system. To pass, however, with this fluid, it
is necessary that the matter conveyed should be soluble in it; and when
it is so, we can discover it in the secretion by chemical tests. If
there is any property connected with it, therefore, which shall prevent
this solubility, it probably will prevent the substance from being
secreted. Now, the _phosphoric acid_, abundant in urine, must in this
mode counteract the secretion of mercury in any form of preparation, by
forming with it a compound, insoluble, and to which the slight excess of
acid cannot communicate solubility; the mercury, therefore, existing in
the circulating mass, when brought in the course of the circulation to
the secreting vessels of the kidneys, will not pass through their whole
course; but if conveyed so far as to be combined with _phosphoric acid_,
will, from this combination, be incapable of being conveyed onwards, and
will therefore be retained in the composition of that part of the blood
which does not enter into the secretion, but returns into the
circulation. It must be discharged by some other emunctory; a portion of
it appears, from some facts, to pass off by the insensible perspiration;
but the _tenuity_ of this secretion, if the term may be employed, must
be unfavourable to this mode of discharge. The salivary secretion is one
by which it may be more easily transmitted; and this transmission may
even be facilitated by the affinity exerted to the _Oxide of Mercury_ by
the _Muriatic Acid_, the _Soda_, and _Ammonia_, which are the chief
saline ingredients in saliva; for it deserves to be remarked that triple
compounds of these substances are, to a certain extent, soluble in
water; and if the _Mercury_ is thus secreted, it will of course
stimulate the secreting vessels through which it passes, and increase
the discharge.
Sir Gilbert Blane[168] has lately advanced another hypothesis to account
for the effects of mercury as a sialogogue; he considers the salivary
glands as one of the outlets for the _ramenta_ of the bones, for by
analysing the saliva we discover the principles of which they consist;
indeed the osseous matter not unfrequently concretes on the teeth, and
sometimes on the salivary ducts, in the form of what is called _Tartar:_
“does not this fact,” says Sir Gilbert Blane, “in some measure account
for these glands being the parts upon which determination is made by the
operation of mercury, which consists in exciting an active absorption of
solid parts, as I have elsewhere observed?”[169]
But do not the kidneys, and other excretory glands also furnish outlets,
through which the _detritus_ of the body is eliminated. How does it
happen, therefore, that the kidneys are not as equally affected as the
salivary glands by the action of mercury? In the present state of our
knowledge it will be more prudent to rest on the phenomenon as an
ultimate fact, than in attempting to ascend higher in the scale of
causes, to involve ourselves in impenetrable darkness.
During the prevalence of the theory which attributed to _Nitric acid_
all the antisyphilitic powers of mercury, it was even maintained that
this acid also excited ptyalism; experience however has disproved the
effects thus attributed to it, and no one attempts to support its
pretensions, as a sialogogue, except indeed as it may perchance, by its
acrid qualities, influence the excretory ducts of the glands,
externally, in the act of being swallowed.
It has very lately been stated by Dr. Macleod,[170] that the
_Hydro-cyanic acid_ occasionally produces soreness of the gums, and a
disposition to ptyalism; this, if true, is a very remarkable fact, and
well deserves attentive consideration.
Some theorists may, perhaps, be inclined to consider certain Nauseating
Medicines as possessing sialogogue properties. It cannot be denied that
an increased discharge of saliva will take place during the operation of
such remedies, but it is very transient, and can never be rendered
available to any therapeutic object. I shall however have occasion to
refer to this fact hereafter, and to the inference deduced from it by
Dr. Eberle, in explanation of the effect of nauseating medicines in
promoting the operation of Mercury.
ERRHINES, or STERNUTATORIES:
Substances which, by direct application to the pituitary membrane,
occasion a discharge from the nostrils either of a mucous or serous
fluid. This class contains several different species, whose operation
varies in intensity, as well as in duration.
Errhines have been regarded as useful in consequence of the evacuation
they occasion, but in this respect their value has been greatly
over-rated; it has been stated, that they diminish the quantity of fluid
circulating in the neighbouring vessels, and even extend their influence
to all the branches of the external carotid; and Dr. Cullen says that he
has, apparently from this operation, known head-ache, pain of the ear,
and some cases of ophthalmia, cured or relieved by the use of Errhines.
There can be no doubt that local stimulants of this kind will frequently
remove pain from the head and neighbouring parts, but not merely by
occasioning vascular depletion, as Dr. Cullen supposed, but by a
stimulant operation conveyed through the medium of nervous
communication, or contiguous sympathy.
Dr. Cullen has moreover supposed, that these substances may be useful in
preventing apoplexy or palsy. Morgagni[171] however relates a case in
which sneezing induced a fatal attack of this disease; and Van
Swieten[172] has satisfactorily shewn, that continued paroxysms of
sneezing tend to load the vessels of the head with blood; for the
violent contraction of the chest impedes, for a time, the passage of the
blood through the lungs, and therefore obstructs the return of the
venous blood from the brain, the vessels of which are in consequence
greatly distended; the face therefore reddens and becomes turgid, the
eyes are suffused with water, and appear full and distended. Its
occasional dangerous violence is said to have given origin to the
benediction so universally bestowed on those who sneeze.[173]
It has been a subject of popular inquiry, how far the habitual use of
Snuff may prove beneficial or injurious; and whether the habit, when
once fully established, can be discontinued with impunity? It may be
remarked that Snuff, by habitual use, soon ceases to produce the effect
of an Errhine, for which reason its discontinuance cannot, generally, be
regarded as likely to be attended with any danger; in those cases,
however, in which the discharge is perpetuated, a contrary judgment
should be pronounced, for all artificial discharges become
constitutional by long continuance, and can therefore be seldom checked
with impunity. Dr. Cullen states, from experience, that “whenever the
nasal discharge has been considerable, the laying aside the custom of
taking snuff has been productive of evil.”