The two great vascular systems, with red and black blood arise and
terminate, as we have said, in the capillaries, which form in the
lungs, as in all the other parts the limits that separate them, and in
which they are changed from one to the other. There are then evidently
two capillary systems distinct from each other, and which are even
opposite. One, generally spread throughout the body, is the seat of
the change of red blood into black. The other, confined wholly to the
lungs, exhibits an opposite phenomenon; it is in its divisions that the
black blood becomes red.
As the capillaries, which serve for the origin and termination of the
black abdominal blood, are intermixed on both sides with those of the
general capillary system, since in the abdomen they are continued with
the arteries and in the liver give origin to the veins, I shall omit
them in these remarks, and attend only to the general and pulmonary
capillary systems.
These two capillary systems, the first especially, deserve the more
particular consideration, 1st, because the circulation is governed
in it by laws wholly different from those of the other parts; 2d,
because most of the important functions of life take place there, as
secretions, nutrition, exhalations, &c.; 3d, because their small tubes
are affected on many occasions by diseases, as they are the seat of
inflammations, metastases, &c.; 4th, because animal heat is especially
produced in these tubes, &c.
The lowest species of animals have in reality only a capillary
circulation. Their fluids do not move in great masses, in canals which
carry them to all parts of the body, and afterwards bring them back
again. There is only an insensible oscillation of these fluids, in
tubes of the greatest delicacy and number. This kind of circulation is
one of the points of contact, or rather of transition from animals to
vegetables, which, destitute of the circulation with a sensible motion,
have evidently like the zoophytes, that of an insensible motion and of
the capillary vessels.
I shall first examine the general capillary system, and afterwards the
pulmonary.