432 THE FEMALE REPRODUCTIVE ORGANS 



which mark off the outer surface of the detached placenta into 

 macroscopical areas or lobules, the placental cotyledons. 



The distribution of the blood vessels of this part is specially 

 interesting. The arteries enter from the muscular coat of the 

 uterus and follow a spirally tortuous course through the decidua, 

 until they arrive near the surface of the compact layer. Here 

 their lumen suddenly broadens and their walls become relatively 



si 



FIG. 349. THE MARGIN or THE 



HUMAN PLACENTA AT FULL 

 TERM. 



A, the placental margin; D, 

 decidua subchorialis ; Cho, cho- 

 rion; Fib, canalized fibrin; Fi, 

 placental villi ; vi, aborted villi 

 outside of the placenta. (After 

 Minot.) 



xl3 



thin, consequently they are with difficulty distinguished from the 

 veins. These arterial spaces at last turn suddenly and enter the 

 placental tissues, pouring their contents directly into the lumen 

 of the intervillous spaces ; hence these are to be regarded as mater- 

 nal blood spaces. The maternal veins also open directly from the 

 intervillous spaces and, though much less tortuous, they retrace 

 the course of the arteries to the vascular layer of the uterine mus- 

 culature. We may liken the intervillous spaces to an enormously 

 dilated capillary space, a great lake, as it were, within which the 

 chorionic villi are suspended and of which the many maternal 

 arteries form the inlets and the corresponding veins of the decidua 



