_Internally._│_a._ _through the branches of the Thoracic duct_.
„ │_b._ _through the branches of the Vena
│ Portarum_.[132]
_Externally._│_c._ _through the branches of divided
│ blood-vessels_.
„ │_d._ _through the branches of Lymphatics_.[133]
2. _Conveyed by absorption_, WITH DECOMPOSITION, _by which one or
more of its constituents are developed, and pass into the
circulating current_.
II. _By an impulse conveyed through the instrumentality of the
nerves._
III. _By the sympathetic control exerted by the stomach on distant
parts._
IV. _By the operation of contiguous sympathy, or of that which is
excited by the mere proximity and continuity of parts._
And it is important to observe, that these are frequently antagonist
operations, and consequently, that remedies, although they should
occasion the same apparent effects, unless they act through the same
medium, are not SIMILAR agents, but on the contrary, are generally
medicinally incompatible with each other; for an illustration of this
truth, the practitioner may refer to the observations which I have
offered under the history of diuretics.
The difficulty of justly appreciating these phenomena, in every
instance, has furnished a powerful objection against the validity of any
classification of medicinal substances which is founded on their
supposed modes of operation; and it must be acknowledged that, if we are
unable to assign to remedies their primary action, or to distinguish
this from their more obvious, though perhaps secondary effects, we shall
frequently be compelled to place similar medicines under opposite heads,
and to include those of very dissimilar characters under the same
artificial division; an error which has contributed more generally to
embarrass and misguide our practice than any other therapeutical
fallacy, and it was the conviction of this truth which induced me to
introduce the present chapter, and to impress the importance of its
subject upon the attention of my practical readers.
It is probable that, in philosophical strictness, no two medicines in
our Materia Medica are perfectly similar, although they recede from each
other by such insensible shades of gradation that we may with practical
advantage admit their parallelism; at the same time, it must be ever
kept in remembrance, that _those Medicines only are practically similar,
whose operations have been found by experience to continue similar under
every condition of the human body; and which, moreover, owe such
similarity to modes of operation which are compatible with each other,
and consonant with the general indications of cure_.
The importance of admitting this proposition will be frequently
illustrated in the sequel; and it may be observed in this place, that
every classification in which it is not recognised as a leading
principle, must be as imperfect in its execution, as it will be unjust
and erroneous in its views.
Before I proceed to any farther discussion upon the present subject, it
will be necessary to offer a synoptical view of an arrangement of
medicinal bodies founded upon the basis of their operations, in order
that I may be better enabled to illustrate the observations which it is
my intention to introduce: for this purpose I shall present the reader
with three different classifications of this kind; the first being that
proposed by Dr. Cullen,[134] and which is now admitted to rest on
principles nearly altogether false, but the investigation of which will
afford many useful lessons of practical importance; the second
classification is by Dr. Young;[135] and the third is that proposed by
Dr. Murray,[136] which, from its simplicity and strict conformity with
the views I intend to offer, will be adopted as being the most eligible
for the occasion.