784 HANDBOOK OF PHYSIOLOGY. 



diately described, the tnembrana decidua, so called on account of its 

 being discharged from the uterus at birth. 



Formation of the Placenta. 



The mucous membrane of the human uterus, which consists of a 

 matrix of connective tissue containing numerous corpuscles, and is lined 

 internally by columnar ciliated epithelium, is abundantly beset with 

 tubular glands, arranged perpendicularly to the surface (fig. 481). These 



Fig. 481. Section of the lining membrane of a human uterus at the period of commencing 

 pregnancy showing the arrangement and other peculiarities of the glands, d, d, d, with their 

 orifices, a, a, a, on the internal surface of the organ. Twice the natural size. 



follicles are very small in the unimpregnated uterus; but when examined 

 shortly after impregnation, they are found elongated, enlarged, and 

 much waved and contorted toward their deep and closed extremity, 

 which is planted at some depth in the tissue of the uterus, and may 

 dilate into two or three closed sacculi. 



The glands are lined by columnar ( (?) ciliated) epithelium and they 

 open on the inner surface of the mucous membrane by small round ori- 

 fices set closely together (, , fig. 481). 



On the internal surface of the mucous membrane may be seen the 

 circular orifices of the glands, many of which are, in the early period of 

 pregnancy, surrounded by a whitish ring, formed of the epithelium 

 which lines the follicles. 



Coincidently with the occurrence of pregnancy, important changes 

 occur in the structure of the mucous membrane of the uterus. The 

 epithelium and sub-epithelial connective tissue, together with the tubu- 

 lar glands, increase rapidly, and there is a greatly increased vascularity 

 of the whole mucous membrane, the vessels of the mucous membrane 

 becoming larger and more numerous; while a substance composed chiefly 

 of nucleated cells fills up the interfollicular spaces in which the blood- 

 vessels are contained. The effect of these changes is an increased thick - 

 nes, softness, and vascularity of the mucous membrane, the superficial 

 part of which itself forms the membrana decidua. 



The object of this increased development seems to be the production 



