Exhalation and secretion are two functions, analogous in this, that
both of them separate from the blood fluids differing from it, and pour
them upon surfaces where they serve different uses. But the following
are their differences.
1st. In exhalation, there is no intermediate organ between the arteries
and the exhalants; a capillary net-work alone separates them; whilst
on the contrary there is always an intermediate organ between the
excretories and the arteries; it is in this organ that the capillaries
are found, in which the second begin and the first terminate. 2d.
The organized machines which elaborate the secreted fluids are then
much more complicated than those which separate the exhaled fluids.
Thus the bile, the urine, the saliva, &c. differ on the one hand
essentially from the blood, and are on the other much compounded;
whilst the serum, &c. closely resembles some parts of the blood, and
is but slightly compounded, containing but few elements. This double
distinctive character of the two kinds of fluids appears to me to be
very striking. 3d. The exhaled fluids are poured out by an infinity
of small tubes separate from each other; the secreted fluids, on the
contrary, are collected in one or more principal tubes that pour them
on the surface where they terminate. 4th. The exhaled fluids re-enter
in great part into the circulation, after having been thrown out; the
secreted fluids, on the contrary, appear to be especially destined
to be rejected. 5th. Many parts receive the first fluids; they are
deposited upon the serous, mucous, synovial and cutaneous surfaces,
in the cellular texture and even in all the organs of nutrition. The
mucous and cutaneous surfaces, the first especially, are the only ones
upon which the others are poured out.
It follows from all these considerations, that the exhaled fluids, as
the fat, the serum, the synovia, the marrow, &c. differ essentially
from the secreted fluids, as the bile, the urine, the saliva, the
mucous, prostate, spermatic, pancreatic fluids, &c. This difference
appears to have struck a great number of authors; yet most of them
have made use of the term secretion to express the separation of
the exhaled fluids from the mass of blood. I think that there is
much analogy between exhalation and secretion. In both, there is the
capillary system, as I have said, between the vessel that brings and
that which carries away; but the capillary system is certainly arranged
very differently in a gland, from what it is in a serous surface; for
example, wherever there is exhalation, there is certainly nothing but
the capillary system; but where there is secretion, the secretory
organ is too considerable not to admit of something more. Besides, by
trusting to inspection, and without wishing to examine the intimate
nature of the organs, it is evident that where there is secretion,
there is a gland, and that this gland is wanting where there is
exhalation.