THE PLACENTA. 



95 



Fig. 55. 



the muscular wall of the uterus, decidua vera, and that which is in contact with 

 the ovum, decidua rejlexa. The decidua does not extend into the neck of the 

 uterus, which after conception is closed 

 by a plug of mucus. The decidua vera 

 is perforated by the openings formed 

 by the enlarged uterine glands, which 

 become much hypertrophied and de- 

 _veloped into tortuous tubes. It con- 

 tains at a later period n umerous arteries 

 and venous channels, continuous with 

 the uterine sinuses, and it is from it 

 that the uterine part of the placenta is 

 developed. The portion of the de- 

 cidua vera which takes part in the for- 

 mation of the placenta is called "de- 

 cidua serotina." 



The decidua reflexa is shaggy on its 

 outer aspect, but smooth within. The 

 vessels which it contains at first dis- 

 appear after about the third month; 

 about the fifth or sixth month the 

 space between the two layers of the 

 decidua disappears, and towards the 

 end of pregnancy the decidua is trans- 

 formed into a thin yellowish mem- 

 brane, which constitutes the external 

 envelope of the ovum. 



The Placenta is the organ by which 

 the connection between the foetus and 

 mother is maintained, and through 

 which blood reaches the foetus and is 

 returned to the uterus. It therefore 

 subserves the purposes both of circu- 

 lation and respiration. It is formed 

 of two parts, as already shown, viz., 

 the maternal portion which is devel- 

 oped out of the decidua vera (sero- 

 tina), and the foetal placenta formed by 

 the villous chorion. Its shape in the 

 human subject is that of a disk, one 

 side of which adheres to the uterine 

 walls, while the other is covered by 

 the amnion. The villi of the chorion 

 (or foetal placenta) gradually enlarge, forming large projections " cotyledons" 

 which each contain the ramifications of vessels communicating with the umbi- 

 lical arteries and veins of the foetus. These vascular tufts are covered with 

 epithelium, and project into corresponding depressions in the mucous mem- 

 brane of the uterine walls. The maternal portion of the placenta consists of a 

 large number of cells formed by an enlargement of the vessels of the uterine 

 wall, and conveying the uterine blood into close proximity to the villi of the 

 ioetal placenta, which dip into these cells. The interchange of fluids, necessary 

 for the growth of the foetus, and the depuration of the blood, take place through 

 the walls of these villi, but there is no direct continuity between the maternal 

 and foetal vessels. The arteries open into the placental cells somewhat after the 

 manner of the erectile tissue. The veins anastomose freely with one another, 

 and give rise at the edge of the placenta to a venous channel which runs aroun'3 

 its whole circumference the placental sinus. 



Sectional plan of the gravid uterus, from Wagner, 

 in the third and fourth month, a. Plug of mucus in 

 neck of uterus, b. Fallopian tube. c. The decidua 

 vera ; c 1 . The decidua vera passing into the right 

 Fallopian tube. The cavity of the uterus is almost 

 completely occupied by the ovum, e e. Points of the 

 reflection of the decidua reflexa (in nature the united 

 deciduae do not stop here, but pass over the whole 

 uterine surface of the placenta), g. Supposed allan- 

 tois. h. Umbilical vesicle, i. Amnion. k. Chorion, 

 covered with the decidua reflexa. d. Cavity of the 

 decidua. f. Decidua serotina, or placental decidua. 



