474 



AX AMIUUCAX TEXT-HOOK OF PHYSIOLOGY. 



Amnion. 



i 



i 'horion. 



i 



are fetal vessels coming from the embryonic structure, the allantois. They 

 comprise the branches and uniting capillaries of the two allantoic or umbilical 

 arteries, and the one (at first two) allantoic or umbilical vein. They are 

 especially well developed within the villi. As growth proceeds, the chorion 

 comes into close contact with the deeidua reflexa, and, as the latter disappears, 

 with the deeidua vera; this portion of it is called chorion Iceve. In the region 

 of the deeidua serotina it enters into the formation of the placenta, and is 

 here called chorion frondosum. 



The Placenta. — The placenta (Fig. --7), or organ of attachment of mother 

 and fetus, is a disk-shaped body, approximately 20 centimeters (7—8 inches) in 

 diameter, attached to the inner surface of the uterine wall, usually either upon 

 the dorsal or the ventral side, and connected by the umbilical cord with the 



navel of the ictus. It consists of 

 a maternal part, the modified 

 deeidua serotina, and a fetal part, 

 the modified chorion, intimately 

 united together. The modifica- 

 tions of the serotina consist of a 

 degeneration of the superficial 

 layers of the mucosa, especially of 

 the epithelium and the glands, and 

 the development of very large 

 irregular sinuses at the surface, 

 into which the uterine arteries and 

 veins freely open. It is a disputed 

 question among histologists whether 

 the sinuses an- maternal or fetal in 

 origin, or really spaces between 

 maternal and fetal tissues. The 

 modifications of the chorion con- 

 sist of a great increase in length 

 and complexity of branching of 

 the villi, a greai development of 

 their contained blood-vessels, and 

 a linn attachment of their tips to 

 the uneven surface of the serotina, 



so that their branches come to float 

 Pig. 228. Diagram ol the placenta (Schafer) : s, pla- 

 centa) sinuses, into which project the fetal villi, con- freely within the uterine sinuses 

 taining tin- nil fetal > otina; m. 1 . i 1 .1 1 ■ • 11 1 



spongy layer, and m, muscular layer, of the uterus; a, :iml to De bathed in uterine blood 

 uterine artery, and v, uterine vein, opening into tin- (Fig. 228). The analogy between 

 placental sinuses. . ,. . , .... . 



the mammalian placental villi and 



the gills of a fish, also highly vascular and floating in liquid, is striking. 

 We shall see later that the analogy is not only morphological but also 

 physiological, inasmuch as the villi have important respiratory functions. 

 The bulk of the placenta is this intravillous portion, of spongy consistence, 



