ACTION OF THE FEMALE. 003 



749. The formation of the Placenta commences by the penetration of the 

 ramified villi, or filamentous processes of the Chorion, into the tubuli of the 

 Decidua ; the villi thus serve as roots, which suck up and convey to the 

 embryo the nourishment secreted for it by the maternal structures. The 

 mode in which these villi, at first consisting merely of cells, become connected 



with the vessels of the Foetus, will be explained hereafter ( 704). This, 



the earliest and simplest mode by which the FoBtus effects a new connection 

 with the parent, is the only one in which it ever takes place in the lower 

 Mammalia, which are hence properly designated as " non-placental," rather 

 than as ovo-viviparous ( 55). In the higher Mammalia, however, there soon 

 occurs a great extension of the vascular tufts of the foetal Chorion, at certain 

 points ; and a corresponding adaptation, on the part of the Uterine structure, 

 to afford them an increased^ supply of nutritious fluid. These specially-pro- 

 longed portions are scattered, in the Ruminantia and some other Mammalia, 

 over the whole surface of the Chorion, forming what are termed the Cotyle- 

 dons ; but in the higher orders, and in Man, they are concentrated in one 

 spot, forming the Placenta. In some of the lower tribes, the maternal and the 

 foetal portions of the Placenta may be very easily separated ; the former con- 

 sisting of the thickened Decidua ; and the latter being composed of the pro- 

 longed and ramifying vascular tufts of the Chorion, dipping down into it. 

 But in the Human Placenta, the two elements are mingled together through 

 its whole substance. On looking at its Foetal surface, we perceive that the 

 umbilical vessels diverge in every direction from the point at which they 

 enter it ; and their subdivisions ramify very minutely, forming a large part of 

 its substance. The terminal ramifications are represented by Dr. J. Reid* as 

 having the form represented in Fig. 23, (Plate I.), each consisting of an artery 

 and vein bound up together ; thus closely resembling the arrangement of the 

 vessels of the gills in aquatic animals. By Weber, however, a somewhat 

 different description of the terminations of the foetal vessels is given ; each 

 villus being represented by him as consisting of a capillary vessel communi- 

 cating with the artery and vein, and making several turns upon itself, so as to 

 form a series of loops. It is of little practical importance which statement is 

 the most correct ; since the essential fact, that each villus contains the termi- 

 nal connecting branch of an artery and a vein, is recognized in both.t The 

 Maternal portion of the Placenta may be regarded, according to Dr. J. Reid, 

 as consisting of a large sac formed by a prolongation of the inner coat of the 

 Uterine vessels ; against the foetal surface of this sac, the tufts just described 

 may be said to push themselves, so as to dip down into it, carrying before 

 them a portion of its thin wall, which constitutes a sheath to each tuft. In 

 this manner, the whole interior of the placental cavity is intersected by nume- 

 rous tufts of foetal vessels, disposed in fringes, and bound down by reflexions 

 of the delicate membrane that forms its proper wall; just as the intestines are 

 held in their places by reflexions of the peritoneum that covers them. This 

 view was suggested to Dr. R. by the very interesting fact, thatt he tufts of 

 foetal vessels not unfrequently extend beyond the uterine surface of the Pla- 

 centa, and dip down into the uterine sinuses ; where they are still covered, 



* Edinburgh Med. and Surg. Journal, Jan., 1841. 



| By Mr. F. Renaud, the variation in the appearance of the placental tufts is attributed 

 to the degree of imbibition which has taken place through the enveloping membrane. 

 When examined without either injection or immersion in fluid, each tuft is seen to con- 

 tain a long convoluted capillary, originating in the umbilical artery and terminating in 

 the vein. In its normal condition, this seems to transmit but one row of blood corpus- 

 cles; but several may be forced along it by the aid of a little pressure. When immersed 

 in fluid, the tuft becomes distended into the form represented by Dr. Reid, owing to the 

 imbibition of fluid. This may probably be their condition whilst in action. Edinburgh 

 Monthly Journal, March, 1843. 



