242 THE UROGENITAL SYSTEM 



decidua vera, where there are only thin-walled partitions between the en- 

 larged glands (Fig. 246). Following the birth of the child, the tension of 

 the umbilical cord and the 'after pains' which diminish the size of the uterus, 

 normally complete the separation of the decidual membranes from the wall 

 of the uterus. The uterine contractions serve also to diminish the size 

 of the ruptured placental vessels and prevent extensive hemorrhage. 

 From the persisting portions of the spongy layer and from the epithelium 

 of the glands, the tunica propria, glands, and epithelium of the uterine 

 mucosa are regenerated. 



The decidual membranes, and the structures attached to them when 

 expelled, constitute the 'after birth.' The placenta usually is everted 

 so that its amniotic surface is convex, its maternal surface concave. It 

 is composed of the amnion, chorion frondosum, chorionic villi with inter- 

 villous spaces incompletely divided by the septa into cotyledons, and 

 includes on the maternal side the basal plate and a portion of the spongy 

 layer of the decidua basalis. Near the center of the placenta is attached 

 the umbilical cord, and at its margins the placenta is continuous with the 

 decidua vera and the remains of the chorion Iseve and decidua capsularis. 



Placentation. Except the egg-laying monotremes, all mammals form some kind of 

 placenta. In the simplest type (Ungulates) the chorionic villi fit into crypts of the uterine 

 mucosa; as the two surfaces are merely apposed, the endometrium is not cast off at birth. 

 The highest type, as in Rodentia and Primates, is characterized by a partial destruction of 

 the uterine mucosa, so that the chorionic villi, dangling in cavernous spaces, are bathed by 

 the maternal blood which issues from eroded vessels; due to intimate fusions, the mucosa 

 is largely lost at birth. Intermediate conditions are found in the Carnivora; in this group 

 there is a chorionic invasion and erosion of the endometrium, yet the maternal blood circu- 

 lates within intact vessels. 



Position of the Placenta in Utero and its Variations. The position of the placenta is 

 determined by the point at which the embryo is implanted. In most cases it is situated on 

 either the dorsal or ventral wall of the uterus. Occasionally it is lateral in position, and, 

 very rarely (i in 1600 cases), it is located near the cervix and covers the internal os uteri, 

 constituting a placenta prcevia. A partially or wholly duplicated placenta, or accessory 

 (succenturiate) placentas may be formed from persistent patches of villi on the chorion 

 laeve. Cases have been observed in which from three to seven subdivisions of the placenta 

 occurred. 



Gross Changes in the Uterus. During pregnancy the uterus enlarges enormously, 

 due chiefly to the hypertrophy of its muscle fibers, and the fundus reaches the level of 

 the xiphoid process. After birth, it undergoes rapid involution; at the end of one week it 

 has lost one-half its weight, and in the eighth week the return is complete. The mucosa 

 is regenerated in two or three weeks from the remains of the spongy layer (Fig. 246). 



