The fluids once absorbed on the different surfaces of which we have
spoken, are carried by a successive motion to the common trunks, which
transmit them to the black blood.
We know not the laws of this motion. It is evident from many
observations formerly made, that it has much analogy with the motion
of the venous blood; but it is also distinguished from it by some
differences.
It appears to be in general more slow. The thoracic duct opened when it
is full of chyle, does not throw out its fluid as far as a vein of the
same size.
The motion of the lymph does not seem to be subject to a reflux in the
neighbourhood of the heart, like the venous blood. For example, the
venæ cavæ, jugulars, &c. are so much the more dilated, in proportion
to the obstacles the lungs have opposed to the return of the blood.
Now in injecting the thoracic duct, I have never observed between its
dilatation and contraction, and the state of the pulmonary organ, any
kind of relation. On the other hand, we never find this duct full
of lymph, as we find the veins full of blood, when an obstacle has
interrupted the motions of the fluid in the last moments.
How happens it, that in the reflux that produces the venous pulse of
the jugulars, the blood does not enter into either absorbent trunk? The
valves, arranged to prevent the entrance of that, which in a natural
state, flows towards the heart, are evidently useless here. We can
clearly attribute this phenomenon only to the relation existing between
the orifice of these trunks and the black blood, as the orifice of the
larynx, foreign by its vitality to external bodies, repulses every
fluid but the air. Blood is never found in the thoracic duct.
There is in the venous blood an evident continuity of motion, from the
capillary system to the heart; it is from this system that it goes,
to be propagated, if we may so say, to that organ. The motion of the
lymph, on the contrary, is incessantly interrupted by glands, each
of which, as I have said, exhibits really in relation to the vessels
that enter and go out of it, a small capillary system. At each gland
the motion necessarily changes its impulse; now as the state of these
glands is susceptible of many varieties, we can easily conceive,
that the motion of the fluids circulating in the absorbent system,
necessarily presents a great number of them; that it may be rapid
in one part, very slow in another, regular here, there irregular,
&c. Hence we must not be surprised if we find some absorbent vessels
dilated, whilst those of the neighbourhood are hardly perceptible.
There is indeed a kind of variety in the veins, but it always has its
source in the origin of these vessels, and never in their course, as
takes place in the absorbents.
The continuity of the venous blood and the frequent interruptions of
the lymph should establish differences not only between the motions of
the two kinds of vessels, but also in the composition of the fluids.
The first is necessarily everywhere the same; the second may vary at
every gland, and take new modifications at each of those through which
it passes.
I should be disposed to think that the insensible contraction of which
the small capillary system of each gland is capable, would aid the
motion of the lymph, by diminishing the course that this fluid must
take, without a new impulse, from the origin of the absorbents to the
black blood, if these organs were wanting. In fact, we know that in the
extremities where there are much fewer glands, there are more frequent
effusions than in the trunk where the absorbents pass through them at
every instant; I speak of those effusions which ought evidently to be
attributed to the want of circulation of the lymph, as those arising
from compression, too long standing, &c. and not those that depend on
an increased exhalation, like those after organic affections.
We see, from what I have said thus far, that we have only a few
disconnected views upon the motion of the lymph; that of the veins,
though still requiring much research, is yet more known; but in order
to give a perfect knowledge of these subjects, the first especially,
many experiments and much further labour are necessary.