I use an analogy as introduction, to which I shall appeal later :
What are the functions of the rhythmic contractions of the uterus during pregnancy? It is granted that these exist and are so marked in twin pregnancies as to cause oppressive flushing of the face in the later months. A number of crude mechanical theories with unsound bases have been advanced as explanations. My explanation has, I think, not been advanced by any one else.
A limiting membrane separates the placental sinuses from the uterine sinuses. Through this limiting membrane, the nutrition of the fetus passes in, and the waste passes out. The nutrition is presented on the outside of this membrane by the blood stream in the uterine sinuses. Considering the small area of the inlet arteries and the vast area of the sinuses, the force conveyed from the heart has not enough power to drive the blood