ACTION OF THE FEMALE. 



705 



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 digitated aspect represented in Fig. 23, (Plate I.) ; but 

 this is one of the more complex forms. In its simplest character, each 

 villus is cylindrical or nearly so ; and the digitated villi are only solitary villi 

 grouped together at the extremity of a primary branch. Each villus contains 

 a capillary vessel, which forms a series of loops, communicating with an 

 artery on one side and with a vein on the other; but the same capillary may 

 pass into several villi, before re-entering a larger vessel. The vessels of the 

 villi are covered, as in the Chorion, by a layer of cells (Fig. 280y), inclosed 



Fig. 280. 



Fig. 281. 



Portion of the external membrane, 

 with the external cells, of a placental 

 villus: a. cells seen through the mem- 

 brane ; 6, cells seen from within the 

 villus; e, cells seen in profile along the 

 edge of the villus. 



Extremity of a placental villus : a, exter- 

 nal membrane of the villus, continuous with 

 the lining- membrane of the vascular system 

 of the mother ; 6, external cells of the villus, 

 belonging to the placental decidua ; c, c, ger- 

 minal centres of the external cells ; d, the 

 space between the maternal and fetal por- 

 tions of the villus; e, the internal membrane of 

 the villus, continuous with the external mem- 

 braneof the chorion;/, the internal cellsof'he 

 villus, belonging to the chorion ; g, the loop of 

 umbilical vessels. 



in basement-membrane (e) ; but the fo3tal tuft thus formed is inclosed in a se- 

 cond series of envelopes (a, b, c), derived from the maternal portion of the 

 Placenta, a space (d) being left, however, between the two, at the extremity 

 of the tuft. 



923. Whilst the foetal 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 certain extent over the branches and stems of the tufts. Between 

 this membrane, or wall of the enlarged decidual vessels, and the internal mem- 

 brane of the villi, there still remains a layer of the cells of the decidua. "t In 

 this manner is formed the Maternal portion of the Placenta, which may be re- 

 garded in its adult state (as was well pointed out by Dr. J. Reid) in the light 

 of a large sac formed by a prolongation of the inner coat of the Uterine vessels ; 

 against the fcetal surface of which sac, the tufts just described may be said to 

 push themselves, so as to clip down into it, carrying before them a portion of 

 its thin wall, which constitutes a sheath to each tuft. Now as every extension 



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



j- Goodsir's Anatomical and Pathological Observations, p. 60. 



