266 



HAND-BOOK OF PHYSIOLOGY. 



To understand the manner in which ihe foetal and maternal blood- 

 vessels come into relation with each other in the placenta, it is necessary 

 briefly to notice the changes which the uterus undergoes after impregna- 

 tion. These changes consist especially of alterations in structure of the 

 superficial part of the mucous membrane which lines the interior of the 

 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. 



FOKMATION 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- 



FIG. 425. 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. 



dantly beset with tubular glands, arranged perpendicularly to the surface 

 (Fig. 425). These follicles are. very small in the unimpregnated uterus; 

 but when examined shortlv after iirmregnation, they are found elongated, 



FIG. 426. Two thin segments of human decidua after recent impregnation, viewed on a dark 

 ground; they show the openings on the surface of the membrane. A is magnified six diameters, and 

 twelve diameters. At 1, the lining of epithelium is seen within the orifices, at 2 it has escaped. 



enlarged, and much waved and contorted toward 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. 425). 



