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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.
We have learned in Section 144 that the mucous lining of the small intestines is crowded with millions of little appendages called villi, meaning "tufts of hair." These are only about 1/30 of an inch long, and a dime will cover more than five hundred of them.
The other, and equally important, task of the villi, the majority of which dip into the mother's blood, is to transmit substances to and from the embryo. We have traced thus far the earliest steps in the development of the ovum.
In terms of this illustration, the water corresponds to the mother's blood, rich in oxygen, mineral matter, and all other kinds of essential nutriment; 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 interchange of material between mother and child as carried on in the placenta can, perhaps, be made clearer if we compare one of the trunks and its branching villi to a human forearm, hand, and fingers. The hand, we will imagine, is held in a basin of water, in which, by turning on a spigot and leaving the outflow unstopped, we have arranged that the water changes constantly.
Place it in a shallow vessel containing water, and examine the mucous membrane with a lens to find the villi. Make a drawing of this section, showing the coats. Study the connection of the small intestine with the large. Split them open at the place of union, wash out the contents, and examine the ileo-cæcal valve. Observe the size, shape, and position of the kidneys.
The inner, or mucous, surface has a fine, velvety feeling, due to a countless number of tiny, thread-like projections, called villi. They stand up somewhat like the "pile" of velvet. It is through these villi that the digested food passes into the blood. Sectional View of Intestinal Villi.
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.
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.
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.
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