674 



HANDBOOK OF PHYSIOLOGY. 



uterus, and which forms, after a kind of development to be immediately 

 described, the membrana decidua, so called on account of its being dis- 

 charged 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 (adenoid 

 tissue), and is lined internally by columnar ciliated epithelium, is abun- 

 dantly beset with tubular glands, arranged perpendicularly to the sur- 

 face (Fig. 461). These follicles are very small in the unimpregnated 

 uterus, but when examined shortly after impregnation, they are found 

 elongated, enlarged, and much waved and contorted towards their deep 

 and closed extremity, which is implanted at some depth in the tissue of 

 the uterus, and may dilate into two or three closed sacculi (Fig. 461). 



FIG. 460. 



FIG. 461. 



FIG. 461. Section of the lining membrane of a human uterus at the period of commencing preg- 

 nancy 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. 



The glands are lined by columnar ciliated epithelium, and they open 

 on the inner surface of the mucous membrane by small round orifices set 

 closely together (a, a, Fig. 461). 



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 (Fig. 462). 



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 tubular 

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

 the whole mucous membrane, the vessels of the mucous membrane be- 

 coming larger and more numerous; while a substance composed chiefly 

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



