and weighs
from five to six pounds.

At birth, the infant weighs a little more than seven pounds, the usual
range being from four to ten pounds, though these limits are sometimes
exceeded.

Duration of Gestation.--The length of time required for the development
of a human being is usually reckoned as about forty weeks. A more precise
statement places it at about two hundred and seventy-eight days. This
limit is often varied from. Cases have occurred in which a much longer
time has been required, and numberless cases have occurred in which
human beings have been born several weeks before the expiration of the
usual time, as stated. There is some uncertainty respecting the exact
length of the period of gestation, which grows out of the difficulty
of determining, in many cases, the exact time when conception takes
place.

Uterine Life.--The uterine life of the new individual begins with the
impregnation of the ovum, which occurs the instant it is brought in
contact with the zoosperms of the male. While in the uterus, the young
life is supported wholly by the mother. She is obliged to provide not
only for her own sustenance, but for the maintenance of her child. And
she must not only eat for it, but breathe for it as well, since it
requires a constant and adequate supply of oxygen before birth as much
as afterward.

How the Unborn Infant Breathes.--Oxygen and nutriment are both supplied
to it through the medium of an organ called the _placenta_, which is
a spongy growth composed almost entirely of blood-vessels, and is
developed upon the inner wall of the uterus, at the point at which the
ovum attaches itself after fecundation. The growing fetus is connected
with this vascular organ by means of a sort of cable, called the
_umbilical cord_. The cord is almost entirely composed of blood-vessels
which convey the blood of the fetus to the placenta and return it again.
The fetal blood does not mix with that of the mother, but receives oxygen
and nourishment from it by absorption through the thin walls which alone
separate it from the mother's blood.

The umbilical cord contains no nerves, as there is no nervous connection
between the mother and the child. The only way in which the child can
be influenced by the mother is through the medium of the blood, to
changes in which it is very susceptible, as we shall see more clearly
hereafter.

The cord is attached to the body of the child at the point called the
_navel_, being