the decidua, and serve much to hold the two membranes together. They run always in a direction away from the placenta, so that the decidua must have expanded more than this part of the chorion grew, whether as the consequence of its own growth or of the stretching produced by the growth of the ovum is not very certain. In any case it assists in the retention of the chorion to the surface of the decidua about the point of reflexion. At this point we ought, as I have before remarked, to find the limit of the sinus-system, which, after numerous dissections on placenta, both in sitú and recently expelled, I have failed to observe. This margin it is which prevents by its firm attachment any blood which may have entered the intervillal passing between chorion and decidua reflexa. space from We will now suppose that by the means just before described the area of the placenta has gradually been clearly defined, and that the villi are vigorously growing. We thus have a placenta of simple construction:-1st, A membrane (chorion) on the inner side; 2nd, the mucous membrane of the uterus (decidua serotina) on the outer; and, 3rdly, villi passing between these, springing from the chorion, branching as they pass to the decidua, into which their extremities become rather firmly planted. Amongst these villi most probably a small quantity of serous fluid exists to assist osmosis. As pregnancy advances the vessels of the mucous membrane of the uterus, at the seat of the placenta, increase in size, becoming more or less varicose, showing their natural tortuosity very markedly. This is especially noticeable on the external or uterine aspect of the placental decidua, which can be exposed by careful peeling it off the line of future separation (see Appendix, p. 197). This placental decidua from the third to the fifth month of pregnancy is of considerable thickness (about one third of an inch), but as pregnancy advances the more the vessels enlarge and the decidual elements become relatively less. But it must be admitted that at this time, and at all periods DESCRIPTION OF PLATE IV. Fig. 9. a, Main stem of villi attached to c, serotina; b, lateral terminal branch recurrent. |