362 



HUMAN UTERUS AND FETAL APPENDAGES. 



The reverse or uterine surface of the placenta is rough and divided into 

 numerous rounded, oval, or angular portions termed lobes or cotyledons, as stated 

 above. These vary from half an inch to an inch and a half in diameter. The 

 whole of this surface consists of a thin, soft, somewhat leathery investment by 

 the decidual membrane, which dips down in various parts to form the grooves that 

 separate the cotyledons from each other. This layer is a portion of the decidua 

 serotina, which, as long as the parts are in situ, constitutes the boundary between 



si 



FIG. 241. HUMAN PLACENTA AFTER DELIVERY AT FULL TERM. 



A, Vertical section through the margin: D, decidua; vi, aborted villi outside the placenta; Cho, chorion;5&, sinus; 

 Vi, placental villi; Fib, fibrin. B, Portion of A more highly magnified to show the decidual tissue near b: 

 v, blood-vessel; d, decidual cell with one nucleus; d', decidual cell with several nuclei. 



the placenta and the muscular substance of the uterus, but which at the time of 

 labor becomes split asunder, so that, while a portion is carried off along with the 

 placenta and constitutes its external membrane, the rest remains attached to the 

 inner surface of the uterus. If a placenta is cut through, if is found to consist of 

 a spongy mass containing a large quantity of blood and bounded by two mem- 

 branes, each less than a millimeter thick; the upper one is the chorion, covered 

 by the still thinner amnion, and greatly thickened where the vessels lie in it; the 

 lower one is the decidual tissue, together with the ends of the villi imbedded in 



