In addition to the nutritive substances in the mother’s blood, such as albumen, iron and fat which are so altered by cell action as to be absorbable through the villi, certain protective substances as the anti-toxines of diphtheria, tetanus, colon and typhoid bacilli are evidently transmitted from the maternal to the fetal circulation. It is claimed by some authorities that pathogenic organisms, for example, anthrax, pneumonia and tubercle bacilli, may be transmitted from mother to fetus, but the reported cases are so rare that the accepted belief is that organisms are seldom transmitted, if the placenta is healthy and intact. But, according to Dr. Williams, the transmission of typhoid occurs frequently, though malarial parasites cannot pass through the villous membranes. Only during comparatively recent years has accurate knowledge of the origin and function of the placenta been available. Many varied and interesting beliefs and superstitions gained currency in the past, but all of them were erroneous. The description of the circulation of the blood by William Harvey in 1628 shed considerable light upon this puzzling question concerning the exchange of fuel and ash between the parent and fetal bodies. But a mistaken belief that the maternal blood actually entered and flowed through the fetal vessels resulted from his valuable discovery. [Illustration: FIG. 23.—Maternal surface of the placenta, surrounded by the membranes and cord. (From a photograph taken at Johns Hopkins Hospital.) ] When we examine this interesting structure, the placenta, after it is cast off, we find it to be a flattened, fairly round, spongy mass, eight or nine inches in diameter, about an inch thick where the cord arises and thinning out toward the margin. Continued from the margin are the filmy fetal membranes, which together form a ruptured sac. The rupture in these membranes is the opening through which the amniotic fluid escapes, and the child passes during birth. [Illustration: FIG. 24.—Fetal surface of the placenta showing origin of cord. (From photograph taken at Johns Hopkins Hospital.) ] The placenta weighs about a pound and a quarter, or ⅙ as much as the child, and accordingly varies in size and weight with the baby. The maternal surface (Fig. 23) having been detached from the uterine wall, is rough and bleeding and is irregularly divided into lobes while the inner, or fetal, surface is smooth and glistening and covered with the amnion. The fetal surface (Fig. 24) is traversed by a number of large blood-vessels which converge toward the point of insertion of the umbilical cord, from the vessels of which they really arise. These vessels branch and divide until their termination in the innumerable chorionic villi floating in the lakes of maternal blood.