GO THE STRUCTURE OF THE HUMAN PLACENTA. 



usually described as decidua serotina. About the same time, the 

 allantois bearing the umbilical vessels applies itself to the in- 

 ternal surface of that portion of the chorion opposed to the de- 

 cidua serotina, and the villi of that portion become vascular, as 

 formerly described. The vessels of the decidua enlarge, and 

 assume the appearance of sinuses encroaching on the space for- 

 merly occupied by the cellular decidua, in the midst of which 

 the villi of the chorion are embedded. This increase in the 

 calibre of the decidual capillaries, goes on to such an extent, 

 that finally the villi are completely bound up or covered by the 

 membrane which constitutes the walls of the vessels, this mem- 

 brane following the contour of all the villi, and even passing to 

 a certain extent over the branches and stems of the tufts. Between 

 this membrane, or wall of the enlarged decidual vessels, and the 

 internal membrane of the villi, there still remains a layer of the 

 cells of the decidua. 



From this period, up to the full time, all that portion of decidua 

 in connection with the group of enlarged capillaries, and vascular 

 tufts of the chorion, and which may now be called a placenta, is 

 divided into two portions. The first portion of the decidua, in 

 connection with the placenta, or forming a part of it, is situated 

 between that organ and the wall of the uterus. This is the only 

 portion of the placental decidua with which anatomists have been 

 hitherto acquainted, and I shall name it the parietal portion. It has 

 a gelatinous appearance, and consists of rounded or oval cells. Two 

 sets of vessels pass into it from the uterus. The first set includes 

 vessels of large size which pass through it for the purpose of sup- 

 plying the placenta with maternal blood for the use of the fostus. 

 These may be named the maternal functional vessels of the pla- 

 centa. The second set are capillary vessels, and pass into this 

 portion of the decidua for the purpose of nourishing it. These 

 are the nutritive vessels of the placenta. 



The account given by Mr. Hunter of the manner in which the 

 functional vessels of the placenta pass through this portion of the 

 placental decidua is still doubted by many, notwithstanding the 

 more recent of Mr. Oweivs* dissections, and the observations of 



* O\ven. Palmer's Edition of John Hunter's Works, Vol. iv. 



