758 THE PLACENTA. 



way the characters of a capillary network, become dilated into sinuses, 

 which communicate freely with the vessels in the muscular walls of the 

 uterus. As the original capillary plexus occupied the entire thickness 

 of the hypertrophied decidua, the vascular sinuses, into which it is thus 

 converted, are equally extensive. They commence at the external sur- 

 face of the placenta, where it is in contact with the muscular walls of 

 the uterus, and extend through its whole thickness, quite to the surface 

 of the foetal chorion. 



As the maternal sinuses grow. inward, the vascular tufts of the cho- 

 rion grow outward, and extend also through the entire thickness of the 

 placenta. In the latter periods of pregnancy, the development of the 

 bloodvessels, both in the foetal and maternal portions of the placenta, 

 is so excessive that all the other tissues, which originally coexisted 

 with them, have nearly disappeared. If a villus from the foetal portion 

 of the placenta be examined at this time by transparency, in the fresh 

 condition (Fig. 272) it will be seen that its bloodvessels are covered 

 only with a layer of homogeneous, or finely 

 granular material, about 7 mmm. in thickness, 

 in which are imbedded small oval-shaped nu- 

 clei, similar to those seen at an earlier period 

 in the villosities of the chorion. The placental 

 villus is now, therefore, hardly anything more 

 than a congeries of ramified and tortuous vas- 

 cular loops ; its remaining substance having 

 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. 



Extremity of a FOSTAL ^^ The uterine follicles have at the same 

 TUFT of the human piacen- time lost their original structure, and have 

 become mere vascular sinuses, into which the 

 tufted foetal bloodvessels are received, as the 

 villosities of the chorion were at first received into the uterine fol- 

 licles. 



Finally, the walls of the foetal bloodvessels having come into close 

 apposition with the walls of the maternal sinuses, the two become adhe- 

 rent and fuse together ; so that a time at last arrives when we can no 

 longer separate the foetal vessels, in the substance of the placenta, from 

 the maternal sinuses, without lacerating either the one or the other, 

 owing to the adhesion which has taken place between them. 



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

 which is shown in the accompanying diagram (Fig. 274), representing 

 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 tufts into the substance of the placenta. At 6, 6, is 

 the attached surface of the decidua, or uterine mucous membrane ; and 



