to the fetal circulation. Occasionally, it is interesting to observe,
the placenta performs a very different function, namely, the
protection of the unborn child from diseases that may attack the
mother. It is able to afford such protection, because the coating of
the villi is not permeable to all sorts of substances. In order to
pass through their walls, material must be in solution; solid bodies,
therefore, are denied admission to the fetal circulation. The most
significant result of this restriction is, perhaps, that so long as
the coating of the villi remains intact and healthful, bacteria
cannot gain access to the unborn child. Since in health there are no
bacteria in the mother's blood, this fact has no bearing upon the
average pregnancy; but in those exceptional cases in which typhoid
fever or some other infectious disease appears during pregnancy, it
is gratifying to know that Nature has provided an unusual defense
against infection of the unborn child.

That we do not know all about the interchange of substances between
mother and child must be admitted; but the essential facts, and they
alone are of interest here, have been established beyond contention.
There is no doubt whatever that the mother's blood surrounds the
placental villi but never enters the child. The fetal blood, on the
other hand, is first in the child's body, then in the villi, and then
returns to the child again. It never enters the blood-vessels of the
mother but passes to and from the placenta as long as pregnancy
lasts.

THE UMBILICAL CORD.--This rope-like structure, familiarly known as
the navel-string, which connects the placenta and the fetus, is
approximately twenty inches long; its length, therefore, is
sufficient to permit the newly born child to lie between the mother's