through the membranes of the lungs; and
that by this addition the colour of the blood is changed from a dark to a
light red. Secondly, that water possesses oxygene also as a part of its
composition, and contains air likewise in its pores; whence the blood of
fish receives oxygene from the water, or from the air it contains, by means
of their gills, in the same manner as the blood is oxygenated in the lungs
of air-breathing animals; it changes its colour at the same time from a
dark to a light red in the vessels of their gills, which constitute a
pulmonary organ adapted to the medium in which they live. Thirdly, that the
placenta consists of arteries carrying the blood to its extremities, and a
vein bringing it back, resembling exactly in structure the lungs and gills
above mentioned; and that the blood changes its colour from a dark to a
light red in passing through these vessels.

This analogy between the lungs and gills of animals, and the placenta of
the fetus, extends through a great variety of other circumstances; thus
air-breathing creatures and fish can live but a few minutes without air or
water; or when they are confined in such air or water, as has been spoiled
by their own respiration; the same happens to the fetus, which, as soon as
the placenta is separated from the uterus, must either expand its lungs,
and receive air, or die. Hence from the structure, as well as the use of
the placenta, it appears to be a respiratory organ, like the gills of fish,
by which the blood in the fetus becomes oxygenated.

From the terminations of the placental vessels not being observed to bleed
after being torn from the uterus, while those of the uterus effuse a great
quantity of florid arterial blood, the terminations of the placental
vessels would seem to be inserted into the arterial ones of the mother; and
to receive oxygenation from the passing currents of her blood through their
coats or membranes; which oxygenation is proved by the change of the colour
of the blood from dark to light red in its passage from the placental
arteries to the placental vein.

The curious structure of the cavities or lacunae of the placenta,
demonstrated by Mr. J. Hunter, explain this circumstance. That ingenious
philosopher has shewn, that there are numerous cavities of lacunae formed on
that side of the placenta, which is in contact with the uterus; those
cavities or cells are filled with blood from the maternal arteries, which
open into th