THE PLACENTA. 



657 



FIG. 209. 



and extend through its whole thickness, to the outer surface of the 

 foetal chorion. 



As the maternal sinuses grow inward, the vascular tufts of the 

 chorion grow outward, through 

 all parts of the placenta. In the 

 latter periods of pregnancy, the de- 

 velopment of blood-vessels, both 

 foetal and maternal, in the placenta, 

 is so excessive that all the other 

 tissues, which originally coexisted 

 with them, have nearly disap- 

 peared. If a villus from the fcetal 

 portion of the placenta be exam- 

 ined at this time in the fresh con- 

 dition (Fig. 209), it will be seen 

 that its blood-vessels are covered 

 only with a homogeneous or finely 

 granular layer, about T mmm. in 



thickness, in which are imbedded EXTREMITY OF A FCETAL TUFT, from the placenta 



small oval nuclei, similar to those 

 at an earlier period in the villosi- 

 ties of the chorion. The placental villus is now hardly anything 

 more than a congeries of tortuous vascular loops ; its remaining sub- 

 stance having been absorbed in the excessive 

 growth of the blood-vessels, the abundance and 

 development of which can be shown by injec- 

 tion from the umbilical arteries. (Fig. 210.) 

 The uterine follicles have lost their original 

 structure, and have become vascular sinuses, 

 surrounding the tufts of fcetal blood-vessels. 



Finally, the walls of the fcetal blood-vessels 

 having come into close apposition with those 

 of the maternal sinuses, the two become ad- 

 herent and fuse together ; so that at last the 

 fcetal vessels in the placenta can no longer be 

 separated from the maternal sinuses, without 

 lacerating either the one or the other. 



The placenta, therefore, when perfectly 

 formed, has the structure shown in the accom- 

 panying diagram (Fig. 211), which represents 

 a vertical section through its entire thickness, 

 receiving the umbilical vessels from the foetus through the umbilical 

 cord, and sending out its ramified vascular tufts into the placenta. At 

 b, b is the attached surface of the decidua ; and at c, c, c, c are the 

 orifices of uterine vessels 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 per- 



2K 



at term, in its recent condition. a. a. Capillary 

 blood-vessels. Magnified 135 diameters. 



FIG. 210. 



EXTREMITY OF A FCETAL TUFT 

 of the placenta ; from an in- 

 jected specimen. Magnified 

 40 diameters. 



At a, a is the chorion, 



