ACTION OF THE FEMALE. 



9Y5 



tufts of the foetal chorion, at certain points ; and a cor- 

 responding adaptation, on the part of the uterine struc- 

 ture, to afford them an increased supply of nutritious 

 fluid. These specially prolonged portions are scattered, 

 in the Ruminantia and some other Mammalia, over the 

 whole surface of the chorion, forming what are termed 

 the "cotyledons;" 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 consisting of the thickened decidua ; and the 

 latter being composed of the prolonged 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 

 the foetal surface of the Human placenta, we perceive 

 that the Umbilical vessels diverge in every direction from 

 the point at which they enter it ; and their subdivisions 

 form a large mass of capillaries, arranged in a peculiar 

 manner (Fig. 254), and constituting what are known 

 as the foetal mill. Each villus contains one or more 

 capillary loops, communicating with an artery on one 



Fig. 254. 



Fig. 253. 



The extremity of a Villus 

 magnified 200 diameters. Af- 

 ter Weber. The loop 1 is filled 

 with blood ; the other loop, 2, 

 is empty ; 3 is the margin of 

 the pellucid villus. 



Portion of the ultimate ramifications of the Umbilical vessels, forming the Foetal Villi of the Placenta. 



side and with a vein on the other; but the same capillary may pass into several 

 villi, before re-entering a larger vessel. The capillaries of the villi are covered, 

 as in the chorion, by a layer of cells (Fig. 255, /), inclosed in basement-mem- 

 brane (e) ', but the foetal tuft thus formed is inclosed in a second series of en- 

 velops (a, 5, c), derived from the maternal portion of the placenta (Fig. 256), 

 a space (c) being left, however, between the two, at the extremity of the 

 tuft. 



981. Whilst the footal portion of the Placenta is thus being generated by the 

 extension of the vascular tufts of the chorion, the maternal portion is formed by 

 the enlargement of the vessels of the decidua, between which they dip down. 

 "These vessels assume the character of sinuses; and at last swell out (so to 

 speak) around and between the villi ; so that finally the villi are completely 

 bound up or covered by the membrane which constitutes the walls of the vessels, 

 this membrane following the contour of all the villi, and even passing to a cer- 

 tain 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." 1 In this manner is 



Prof. Goodsir's "Anatomical and Pathological Observations," p. 60. 



