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Henry Gray (1825–1861).  Anatomy of the Human Body.  1918.
 
is also named the non-placental part of the chorion. On the other hand, the villi on that part of the chorion which is in contact with the decidua placentalis increase greatly in size and complexity, and hence this part is named the chorion frondosum (Fig. 28).


FIG. 35– Transverse section of a chorionic villus. (See enlarged image)



FIG. 36– Primary chorionic villi. Diagrammatic. (Modified from Bryce.) (See enlarged image)


The Placenta.—The placenta connects the fetus to the uterine wall, and is the organ by means of which the nutritive, respiratory, and excretory functions of the fetus are carried on. It is composed of fetal and maternal portions.


FIG. 37– Secondary chorionic villi. Diagrammatic. (Modified from Bryce.) (See enlarged image)


Fetal Portion.—The fetal portion of the placenta consists of the villi of the chorion frondosum; these branch repeatedly, and increase enormously in size. These greatly ramified villi are suspended in the intervillous space, and are bathed in maternal blood, which is conveyed to the space by the uterine arteries and carried away by the uterine veins. A branch of an umbilical artery enters each villus and ends in a capillary plexus from which the blood is drained by a tributary of the umbilical vein. The vessels of the villus are surrounded by a thin layer of mesoderm consisting of gelatinous connective tissue, which is covered by two

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