ON THE
OPERATIONS OF MEDICINAL BODIES,
AND ON THE
CLASSIFICATIONS FOUNDED ON THEM.
Medicinal Substances are those bodies, which, by due administration, are
capable of producing certain changes in the condition of the living
system, whereby its morbid actions may be entirely removed, or
advantageously controlled.
In adopting this definition we intentionally exclude those alimentary
substances which are more immediately subservient to the support of
life, and to the repair of that diurnal waste, which the exercise of its
functions so inevitably occasions.
It has been generally supposed, that substances whose application does
not produce any sensible action upon the healthy system, cannot possess
medicinal energy; and, on the contrary, that those bodies which occasion
an apparent effect in health, must necessarily prove active in the cure
or palliation of disease. Under certain limitations we may perhaps
venture to assent to this general proposition; but it cannot be too
forcibly or too frequently impressed upon the mind of the medical
practitioner, that _Medicines are frequently but relative agents_,
producing their effects in reference only to the state of the living
frame; we must therefore concur with Sir Gilbert Blane in stating, that
the virtues of Medicines cannot be fairly essayed, nor beneficially
ascertained, by trying their effects on sound subjects, because that
particular morbid condition does not exist which they may be exclusively
calculated to remove;[127] thus in certain states of debility, _Tonics_
may excite the system when languid, by their sympathetic influence upon
the _primæ viæ_, while in a robust condition of the body, the effects of
the same agents may be wholly inappreciable.
The MODUS OPERANDI of remedies, or the general principle upon which they
effect salutary changes in the morbid states of the body, is involved in
considerable obscurity, and has given rise to much ingenious speculation
and scientific controversy. It would seem that the immediate impression
of a remedy may depend upon mechanical, chemical, or vital agencies; and
that the sanative impulse thus occasioned may either be Absolute, or
Relative;—Primary, or Secondary;—Local, or General;—Direct, or
Sympathetic;—Permanent, or Transient;—thus certain purgatives will
occasion intestinal excretions in every condition of the body, and may
therefore be justly considered as _absolute_ agents; while diuretics,
since they generally require for their success, a certain state of the
living system, may with equal truth be denominated _relative_ in their
operation. That the obvious effect of a remedy may either depend upon
its _Primary_, or upon its _Secondary_ and incidental operation, will at
once be apparent by inspecting the diagram which exhibits the
classification of diuretic medicines; the same scheme will also shew
that remedies may be _local_ or _general_ in their effects, and may
excite an action in distant organs, either by entering the circulating
mass, and being thus brought into contact with their textures;—by
occasioning an impulse conveyed through the nervous system,—or by
exciting a local impression upon the stomach and _primæ viæ_, and thus
arousing their energies through the mysterious medium of
sympathetic[128] communication.
That certain bodies are capable of evading the assimilating functions,
and of entering, unchanged, into the circulating current, either through
the branches of the thoracic duct, or of the _vena portarum_, is a fact
which admits of chemical demonstration; many of the alkaline salts are
thus conveyed to the kidneys, and being excreted from the blood by its
vessels, are to be easily detected in the urine by appropriate reagents;
I have made many experiments upon this subject, and am prepared to state
some results which may perhaps explain the occasional value of such
bodies as medicines. Some essential oils, particular _bitter_
principles,[129] and certain colouring matter,[130] seem also capable of
passing the barriers of digestion, and of circulating to the remote
parts of the body; Mercury, and several of the other metals, would
likewise appear, under certain circumstances, to possess a similar
privilege, and the former to be able moreover to facilitate the
absorption of other bodies with which it may be associated, as I shall
hereafter more fully exemplify.
In some instances, the medicinal body undergoes a partial decomposition
by the digestive organs, _in transitu_, by which some of its
constituents escape into the circulation, while the others are
completely digested, and converted into chyle; this occurs with saline
compounds into which vegetable acids enter as constituents. See _Potassæ
Acetas_. It is also stated in the history of Diluents, that there is
reason to believe that _Water_ may under particular circumstances suffer
decomposition, and transfer its elements for the formation of new
compounds, furnishing _oxygen_ to some, and _hydrogen_ to others. There
is likewise reason to suppose, that in particular conditions of the
digestive functions, a remedy may be at once rendered inert by its
entire decomposition.[131]
That an impression made upon the stomach by a medicinal agent, should be
the means of exciting an action in the distant parts of the machine,
will not appear extraordinary when we consider how universal a sympathy
and control this central organ exercises over every function of the
body; imbued with exquisite and diversified sensibilities,—subjected to
the first and coarsest impressions of our various ingesta,—stretched
occasionally to an enormous extent by the unrestricted indulgence of
appetite,—disturbed by the passions,—exhausted by volition, and
debilitated by intense thinking; in short, assailed by numerous foes
from _without_, and harassed by various revolutions from _within_, can
we feel surprised that the aberrations of this viscus should give origin
to the greater number of maladies with which we are afflicted, or that
those medicinal applications should be effective that are directed for
their cure, through the medium of its sympathies?
A dose of Ipecacuan, by exciting the stomach, will abate both the force
and velocity of the heart in its vital motion, and affect the whole
series of blood vessels, from their origin to their most minute
ramifications, as is evinced by the pallor of the skin under its
operation, as well as by its efficacy in arresting hemorrhage; so the
brain, when disordered by vertigo, frequently derives instantaneous
relief from the administration of a tea-spoonful of æther in a glass of
water. The stomach however is not in every case the medium of sympathy;
a substance may excite a powerful impression upon a distant part, by the
instrumentality of the nerves, without any concurrence of the stomach;
thus, the _Belladonna_, by coming in contact with the _Tunica
Conjunctiva_ of the eye, will occasion immediate dilatation of the
_Iris_, although no other part of the system is in the slightest degree
affected.
But there is yet another mode by which remedies may be made to exert a
sanative effect upon particular organs of the body, through the medium
of what Mr. Hunter called _contiguous_ sympathy, and whose existence
appears to depend upon the mere proximity and contiguity of parts,
without any relation to the distribution of the nerves; thus it is, that
relief is afforded to a deep-seated inflammation, by scarifying the
nearest external surface; while we know from long experience that the
thoracic or abdominal viscera, when similarly affected, receive
corresponding relief from the same topical use of bleeding, blistering,
or fomenting.
With respect to the _Modus Operandi_ of medicines the following
classification may be established.
THE PARTICULAR ORGANS OF THE BODY MAY BE EXCITED INTO ACTION, THROUGH
FOUR DISTINCT AND DIFFERENT MODES OF COMMUNICATION.