If I were to run over the phenomena of absorption in the different ages, sexes, seasons and climates, I could show constantly the differences of organic sensibility always preceding the differences of this function. I shall speak of them in the different ages.
The causes that vary the natural type of the sensibility of the absorbents, are, as in all the other functions, direct or sympathetic; 1st, direct, as when by previous friction on the skin, we excite the absorbents, and force them to act, which they would not have done without this; 2d, sympathetic, as when the absorbents, feeling the affection of a distant viscus, increase or diminish their action, according to the kind of influence they receive. We have spoken of this phenomenon in the sympathies of the different systems.
III. Motion of the Fluids in the Absorbents.
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.