THE PLACENTA. 



625 



Fig. 227. 



with them, become retrograde and disappear almost altogether. If 

 a villus from the foetal portion of the placenta be examined at this 

 time by transparency, in the fresh condition (Fig. 226) it will be 

 seen that its bloodvessels are covered only with a layer of homo- 

 geneous, or finely granular material, -j^Vu f an i ncn i n thickness, 

 in which are imbedded small oval-shaped nuclei, similar to those 

 seen at an earlier period in the villosities of the chorion. The vil- 

 losities of the chorion are now, therefore, hardly anything more 

 than ramified and tortuous vascular loops ; 

 the remaining substance of the villi hav- 

 ing been atrophied and absorbed in the 

 excessive growth of the bloodvessels, the 

 abundance and development of which 

 can be readily shown by injection from 

 the umbilical arteries. (Fig. 227.) The 

 uterine follicles have at the same time 

 lost all trace of their original structure, 

 and have become mere vascular sinuses, 

 into which the tufted fcetal bloodvessels 

 are received, as the villosities of the cho- 

 rion were at first received into the uterine 

 follicles. 



Finally, the walls of the fcetal blood- 

 vessels having come into close contact with the walls of the maternal 

 sinuses, the two become adherent and fuse together ; so that a time 

 at last arrives, when we can no longer separate the fcetal vessels, in 

 the substance of the placenta, from the maternal sinuses, without 

 lacerating either the one or the other, owing to the secondary 

 adhesion which has taken place between them. 



The placenta, therefore, when perfectly formed, has the structure 

 which is shown in the accompanying diagram (Fig. 228), repre- 

 senting a vertical section of the organ through its entire thickness. 

 At a, a, is seen the chorion, receiving the umbilical vessels from the 

 body of the foetus through the umbilical cord, and sending out its 

 compound and ramified vascular tuft& into the substance of the 

 placenta. At b, b, is the attached surface of the decidua, or uterine 

 mucous membrane ; and at c, c, c, c, are the orifices of uterine ves- 

 sels which penetrate it from below. These vessels enter the placenta 

 in an extremely oblique direction, though they are represented in 

 the diagram, for the sake of distinctness, as nearly perpendicular. 

 When they have once penetrated, however, the lower portion of 

 40 



Extremity of FCETAL TUFT of 

 hnmau placenta; from an injected 

 spec. men. Magnified 40 diameters. 



