This structure, capable only of being seen in the great trunks,
the thoracic duct, for example, presents us at first in its common
organization, a layer of dense cellular texture, of the same nature
as that of which we have already so often spoken, of which we shall
speak again, and which is found around the arteries, the veins, the
excretories, under the mucous surfaces, &c. &c. This filamentous
texture, connected only to a certain degree with the vessel,
strengthens it however much, by surrounding it with an external
membrane superadded to that which is peculiar to it. If, as Cruikshank
has done, we turn this duct inside out, and introduce into it a tube of
glass of a diameter a little larger than its own, this last membrane
will break. It is as in the arteries, in which a ligature cuts the
internal membrane and not the cellular. The same phenomenon takes place
from inflation; a much greater effort is necessary then to break the
cellular texture, than to rupture the peculiar membrane of the thoracic
duct.
No fleshy fibre is observed, in an evident manner at least, in the
absorbents. Some authors have admitted that they were there, but
injection contradicts them, even as it regards the thoracic duct. Blood
vessels probably run over the parietes of the absorbents; in ordinary
injections they are often very conspicuous on the thoracic duct. We
know not if there are nerves there; there is but little appearance of
them, if we may judge by the analogy of the veins, which have a great
relation in structure with these vessels.
The internal membrane which forms the peculiar texture of the
absorbents is continuous with that of the veins, and forms with it an
uninterrupted series of small tubes. Delicate, transparent, it is
moistened in the dead body by an unctuous fluid, which is, I believe,
unknown to it in the living, as that of the arteries is to those
vessels. It adheres to the external membrane by a compact cellular
texture, which, as in the veins, is rarely subject to ossification.
Mascagni has however mentioned an instance of it in the absorbents of
the pelvis. But there is another affection analogous to this, which
I have already seen many times in this kind of vessels. Their cavity
often contains a white matter, like plaster, especially on the external
surface of the lungs. Then without any preparation, the absorbents
exhibit almost the appearance which they have when mercury fills them.
The peculiar membrane, forms by its folds, valves similar to those of
the veins, but much more numerous. We find these united two by two,
rarely one exists alone. They leave between them small intervals, very
variable however in extent. Hence it happens that the thoracic duct
can sometimes be injected from above below through its whole extent,
sometimes it receives the fluid only in a short space, according as
the valves are more or less numerous in its cavity; which depends also
much on the relation of their width to the caliber of the vessel, a
relation which varies from the same causes as those assigned for the
veins. Hence it happens, that an absorbent filled with injection does
or does not exhibit in great number those knots, which, as we have
said, indicate valves. Wherever a branch is united to a trunk two of
these folds exist at the place of their junction. This is remarkable
especially in the thoracic duct, which injected from above, presents
a dilatation at the origin of each branch, because in this place the
valves are opposed to the fluid. Not numerous in the superficial system
of the organs covered by the serous membranes, as upon the convexity
of the lungs, and the spleen, they easily allow the passage of the
mercury from one division to another, and their ordinary functions are
supplied there by the great number of the anastomoses.
Their use is the same as in the veins, viz. to permit the ascent of
the fluid, and to prevent its return; but they do not always fully
do this. Injection often without difficulty overcomes some of them.
In dropsies, in which the absorbents are full, if we raise the skin,
we easily distinguish these vessels by their transparency; but soon,
notwithstanding their valves, they become empty, and then cease to be
visible. Different anatomists have forced air, and even other fluids,
into a great number of the lymphatics, by means of the thoracic duct,
and consequently in an opposite direction to their valves. All these
phenomena do not suppose in these vessels, as in their common duct,
varieties in the structure of the valves, in their width, &c. but only
different degrees of dilatation and contraction, degrees that are, as
I have said, independent of structure. In dilatation, the valves close
the caliber less than in contraction.
The valves of the absorbents have the same form and the same
arrangement as those of the veins; they partake, by their constant
exemption from ossification, of the general character of the membrane
from which they arise, and which, by folding, forms them.