Authors have formed very different ideas concerning the exhalants.
We know the decreasing vessels of Boerhaave, and the error loci for
which his imagination created these vessels. Lately all the white
vessels continuous with the arteries have been rejected, and in order
to explain exhalation, recourse has been had only to inorganic pores
in the arterial parietes, through which the fluids transude upon the
organs. Frequent observation of similar transudations upon the dead
body, as those of the bile through the gall-bladder, of the marrow
through the osseous texture which it discolours, &c. is one of the
great supports of this method of explaining the exhalant system. But
we have already many times observed, that these phenomena never take
place during life, when the organic sensibility of the parts refuses
to produce them. Besides, exhalation is evidently subjected to the
influence of the vital forces, since it varies continually in a part,
according as the vital forces of the part themselves vary. Moreover,
if the exhaled fluids escaped through inorganic pores, it would be
necessary that not only the vascular parietes, but also those of the
serous surfaces which receive these fluids, should be perforated
with small holes; why then would not these fluids, of which these
surfaces are the reservoirs, transude into the neighbouring cellular
texture? Let us reject then every opinion that disregards anatomical
observation, and let us endeavour by this observation to ascertain what
the exhalants are.
It is undoubtedly difficult to form a precise idea of these vessels,
their extreme tenuity constantly conceals them from us in a natural
state. Yet by the aid of experiments and accurate reasoning, it
appears to me that we may come at some degree of accuracy.
We have seen that the existence of a capillary system terminating
the arteries, is in the parts where exhalation takes place as in the
others, a thing incontestably proved by experiment with injections, by
inflammations which arise spontaneously, and by those that are produced
at will; so that a serous, cutaneous surface, &c. on which nothing
appears, is covered with an infinity of little vessels suddenly in the
first case, and at the end of a longer or shorter time in the second.
If the injection is not pushed very far, it is confined to the
capillary system; but if it succeeds, it pours from all parts upon
the surface, where exhalation takes place in the ordinary state. This
dew mechanically produced, evidently resembles that which the tonic
force of the parts occasions during life; for, as I have said, if it
was a transudation, there would be extravasation in the neighbouring
textures, whereas nothing is filled from the syringe which propels
the injection to the exhalants that pour it out, except the arteries,
the capillaries and these exhalants. Besides, when there is active
hemorrhage, the capillaries from which arise the exhalants that pour
out the blood, are evidently more full of fluid than ordinary, as I
have already remarked.
From these considerations and many others that will be explained
hereafter in this system, I think that we may consider the exhalants as
arising from the capillary system, by means of which they are continued
with the arteries, which bring them the materials of exhalation.
But to say what is the length of these vessels, what their form, what
course they run, is evidently impossible; it is here that the imaginary
descriptions begin. We distinguish with difficulty their orifices. We
see upon the skin many little pores that evidently form communications
from within to without; but these pores transmit not only the
exhalants, but also the absorbents, the hair, &c. as we shall see in
the dermoid system. 1st. The existence of the exhalants; 2d, their
origin in the capillary system of the part where they are found; 3d,
their termination upon different surfaces, are upon the whole all that
is accurately known.
The mode of origin undoubtedly varies, but we do not know how it takes
place. The exhalants are continued with their capillary net-work, in
such a manner that we cannot say precisely where one finishes and
the others begin. Hence why often in this work, in speaking of these
small tubes, I suppose them to come immediately from the arteries,
and forming the capillaries by their interlacing; this is evidently
sufficient to understand what will be said hereafter.