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 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 knees while the placenta remains attached to the womb. The cord is about the thickness of the thumb and contains three blood-vessels, all filled with fetal blood; in two of them the current is directed toward the placenta, the third carries the blood back to the fetus after it has circulated through the placental villi. In the cord the vessels lie near together and are encased in a jelly-like substance that protects them from injury.
So far as is known; the umbilical cord performs no service other than to link the blood-vessels in the placenta with those in the fetus. Simple as this may seem, it is of paramount importance in maintaining the life of the fetus, for compression of the vessels in the cord would shut off its nutriment. Against such accident, however, perfect provisions have been made; both the amniotic fluid and the jelly-like substance which surrounds the vessels are safeguards which effectually protect the circulation from pressure that might interrupt it.
Frequently, prospective mothers are told they must not "reach up" for fear the cord will become entangled. Such a precaution is quite unnecessary. No matter what the mother does, or does not, the cord will be found around the child's neck at the time of birth in one of every three cases. It is not difficult to understand how this happens. The cord is longer than the uterine cavity and must fall in coils toward the bottom of it. Now, since the fetus is free to move it enters and withdraws from these loops, many times, in the course of pregnancy. Finally, when it takes up a position head downward, as it nearly always does, the head is the part of the fetus which passes through the coil, should one happen to lie in its path. After the head is delivered the physician always feels about the neck to discover whether a loop of cord is there. If it is, he can release it easily. This condition, since it occurs so frequently and since it so rarely produces harmful consequences, should not be considered unnatural.
After the child is born, the physician cuts the cord, and in due time the after-birth is expelled through the same passage as was the child. The expulsion of the after-birth frees the mother of all the tissue derived from the growth of the ovum, for the intricate mechanism that served to nourish and protect the embryo was almost entirely developed from the ovum itself. It is a remarkable provision of Nature that very little of the mother's tissue is cast off at the end of pregnancy; and even this small portion is promptly replaced. By about the sixth week after delivery, the wound which was made by the separation of the fetal sac has completely healed. Meanwhile the mucous membrane that underwent elaborate preparations to receive the ovum, the cavity that was adjusted to its growth, and the muscle fibers that were strengthened to insure its safe entry into the world have all regained their original state. Except for the activity of the breasts, the mother is left in the same physical condition as before she became pregnant.
CHAPTER III
THE EMBRYO
The Development of Form—The Determination of Sex—Twins—The Rate of Growth—The Newborn Infant—Heredity—Maternal Impressions.