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.