378 THE PLACENTA. [Boon iv. 



been and still is much disputed the evidence seems to be in favour 

 of the view that these iutervillous spaces are, in the living body, 

 tilk'd with maternal blood, that they are in reality blood sinuses 

 belonging to the maternal circulation. Since the villi branch out in 

 all directions they present in a thin vertical section the appearance 

 of an irregular labyrinth with many outlying islets, and if before 

 the section is made the intervillous spaces be injected as they may 

 be from the maternal blood vessels, the injection material appears 

 likewise as an irregular labyrinth tilling up the interstices of, and 

 siirruuii<liiiu the islets of, the foetal labyrinth. The placenta is in 

 ta.-t ;i labyrinth of foetal villous tissue fitting into a corresponding 

 labyrinth of maternal intervillous spaces, each branch of a foetal 

 villus projecting into and being bathed by the blood of a maternal 

 sinus. 



Into this labyrinth of blood sinuses blood is poured by the 

 curling uterine arteries, each of which as it passes through the 

 decidual layer on its way from the muscular coat loses most of the 

 elements of its coats until these are reduced to a single layer of 

 epithelioid plates, and suddenly opens by a more or less round 

 mouth into a blood sinus without the intervention of any capil- 

 laries. From the sinuses the blood escapes .by more irregular 

 orifices into veins, which pursuing a straighter though oblique 

 course through the decidual layer, pass to the uterine muscular 

 coat ; much of the returning blood flow's by a vein or rather a 

 plexus of veins, which takes a circular course around the edge of 

 the placenta. 



953. The sinuses of the placental labyrinth appear to be 

 lined by epithelioid plates continuous with the lining of the 

 maternal arteries and veins ; and, though on this point there 

 is difference of opinion, we may probably look upon the sinuses 

 as being greatly transformed maternal capillaries. The body 

 of each villus consists of arteries branching into capillaries, and 

 so ending in returning veins, all supported by an immature 

 though abundant connective tissue ; but the nature of the wall 

 of the villus, that which forms the partition separating the blood 

 of the foetal capillary from the blood in the intervillous space 

 or maternal sinus has been the subject of much controversy. 

 The view which has perhaps the greater support is that the 

 basement membrane or surface of the foetal connective tissue 

 bears two (some say three) layers of epithelium, of which the 

 inner one is foetal, a derivative of the epiblast of the chorion, 

 and the outer one is maternal, having probably the same origin, 

 whatever that may be, as the epithelioid plates lining the rest of 

 the sinus. In any case there can be no doubt that an epithelium 

 of some kind or other does separate the foetal from the maternal 

 blood, and it is worthy of notice that the cell-substance of the cells 

 of this epithelium has the appearance of being ' active ' cell- 

 substance engaged in metabolic labours. 



