'2*>'2 MICROSCOPIC .\\.\To.MY or TUT. ORGANS. 



tissue bands arise, the so-called septa i>!<'< n(<r. These pass 

 between the chorionic villi and separate them into groups or 

 cotyledons. Only at the peripheral part of the placenta do 

 the septa, come into contact with the membrana chorii and 

 fuse with it to form the so-called mbchorionic limiting ri/iy. 



The circulation of blood in the maternal placenta takes 

 place in the following way : numerous arterial branches enter 

 through the muscular coats of the uterus to the outer layer of 

 the placenta uterina. During their tortuous course these 

 vessels lose their muscle cells and elastic elements, so that the 

 thin walls that remain consist only of an endothelial and thin 

 connective-tissue layer, and come to lie directly on the decidnal 

 cells. After branching, the arteries enter the septa placentae, 

 where they empty into the intervillous spaces through openings 

 in the septa. The veins also open into these spaces, so that 

 instead of a capillary system between the arteries and veins we 

 find wide lacunae, which, according to most authors, arise from 

 the superficial blood capillaries of the uterine mucosa, The 

 veins, whose walls, like those of the arteries, have been reduced 

 in thickness, open into the intervillous spaces by comparatively 

 wide orifices, which are more abundant near the middle of the 

 cotyledons. The arteries, on the contrary, open in greatest 

 numbers at the edges of the cotyledons, so that the blood in the 

 intervillous spaces flows from the periphery to the centre of the 

 cotyledons. 



The intervillous spaces thus contain maternal blood, while 

 in the chorionic villi the capillary vessels under the epithelial 

 covering are all of foetal origin. These two vascular systems 

 never communicate directly with one another, and a mixture 

 of foetal and maternal blood never occurs. The diffusion of 

 gases takes place through the walls of the capillaries and 

 through two layers covering the villi. 



D. Vagina and External Female Genitals. 



The wall of the vagina is about 3 mm. thick, and consists 



of four layers : the mucosa, submucosa, muscularis, and fibrosa. 



The mucosa is thrown into transverse folds, the so-called 



