In order to be fully satisfied of the Nature of Milk, it is necessary
to examine into the Manner of its Generation: It seems reasonable to
imagine, that the Chyle, once received into the lacteal Vessels, and
at length mixed with the Blood, is never again let forth with the
same Appearance; only in Women at the Time of Childbirth, when it is
plentifully separated, through the Ramifications of the Arteries, by
the conglomerate Glands of the Breast. There is evidently a great
Agreement between the Milk and the Chyle, in as much as the Chyle
consists of a watry, limpid and gelatinous Fluid, with oily or fat
Globules swimming therein. These Globules are pellucid, and differ both
in Size and Figure; the Reason of its Whiteness is to be imputed to
this: The oily Globules are mixed with the watry ones, in such Manner
that several very smooth Globules are formed, which reflecting the
Rays of Light in right Lines, occasion a white Colour; the same thing
is observable in making Emulsions with oily Seeds, or upon mixing
resinous Essences with Water, or mixing Oyl and Water, and shaking them
well together; in these Cases, the watry and oily Particles, being
thoroughly mixed, occasion such a Superficies as reflects a white
Colour. _Bolin_, and several Authors have proved, that the Milk is
no other than oily or fat Lymph or Chyle, brought with the Blood to
the Breasts, and there deposited in the milky Cells. _Berger_ hath
very well explained the Manner of its Separation in the Breasts. The
whole Substance of the Breasts, in Women giving Suck, is made up of
various Ramifications of Arteries, from the thoracick and mamillary
Arteries, which terminate in oval Cells, or glandulary Follicles;
from hence the Breast swells with many milky Vessels, terminating
in the Nipple; through these the more oily and chylous Parts of the
Blood are derived from the Glands, where it is not only separated, and
received, but gathered and preserved, while the remaining Mass of the
Blood is returned by the Veins and Lymphaticks. These milky Rivulets,
after breaking very small from the Ramifications of the Arteries,
flow together into several larger Trunks, which in their Progress are
united by Insertions of their Parts, in some Places more dilated, in
others streightned, from several Cells and Cisterns, where the Milk
is gathered and preserved, so as always to have a sufficient Quantity
for the Nourishment of the Infant. Lastly, as the Chyle is separated
from the Mass of the Food in the Bowels, not by any Precipitation,
but by Percolation only; and as in the making of Emulsions, the oily
Seeds communicate an oily Milkiness to the Water, and is separated
from the grosser Parts by the Sieve, without the Intervention of any
precipitating Medicine, so the chylous Juice is separated in the Bowels
by gentle Pressure or the Peristaltick Motion, and strained through the
Orifices of the lacteal Vessels, to be thence thrown into the Mass of
the Blood. In like manner, the Milk is barely separated, by straining
the milky Particles from the Blood, through the small Ramifications of
the Arteries in the Glands of the Breasts.