and the fingers are the villi. The blood-vessels in the fingers, to
go a step farther, represent the blood-vessels which exist within the
villi, connecting with the umbilical cord, and passing by that route
to the body of the child. The blood which thus circulates through the
villi, it is important to emphasize, is the child's blood; it cannot
escape through the coating of the villi, just as our blood cannot
escape through the skin of the fingers. Similarly, the mother's blood
cannot enter the child; the two circulations are absolutely separate
and distinct.

It must be noticed, moreover, that the maternal blood not only brings
to the surface of the villi everything the child needs, but it also
takes away the waste products of fetal life. Let us select one of the
foodstuffs necessary for the unborn child, and follow its course so
far as it relates to fetal nutrition. The mother's blood brings
sugar, for example, from her intestinal tract to the surface of the
villi; through the coating of the villi the sugar passes into the
fetal blood, is carried to the fetal heart, and distributed to the
various fetal organs. They burn it, deriving heat and energy, and in
return give off waste products, namely, carbonic acid gas and water,
which are taken up by the fetal blood, borne back to the placenta,
and pass again through the coating of the villi into the mother's
circulation. These waste products are then transported to the
mother's lungs and to her kidneys, and are finally thrown off from
her body. Before the child is born, therefore, the placenta, which is
an aggregation of villi, acts as its stomach, intestines, lungs, and
kidneys.

In every pregnancy the placenta serves in this way as an organ of
nutrition, arranging for the passage of food from the mother's blood