Are the maternal blood-spaces provided with epithelium? Although Turner affirms it, Leopold disproves the existence of such a layer. We must, therefore, consider the blood-spaces as a system of lacunae, commu nicating with one another, and stretching from the chorional to the mater nal placenta; while the inter-cotyledonal spaces are the conduits of the great labyrinth which is filled with the maternal blood.
The arteries of the serotina bring the maternal blood to the blood spaces. Reduced gradually to their endothelial intima, they open di rectly as capillary vessels into the blood lacunae.
At Five Months.—The mucous layer of the serotina is .117 of an inch in thickness, and is situated between the placenta and the muscular coat. The glands are larger and rounder. The compact layer has become thicker, and forms the maternal placenta, which projects into the festal placenta, the inter-cotyledonal net-work. At the summit of the villosities, the point of their adherence to the maternal placenta, the epithelial covering of the villi becomes thinner, or even completely disappears; the body of the villus is directly applied to the cells of the decidua. The inter-cotyledonal net-work does not penetrate half way through the Veal placenta at its centre; while at its edge the net-work goes to the chorion. Thus the cells of the serotina are directly applied to the chorion, either as an elevated cone, on each side of which are villosities and blood-spaces; or as a cone broken into two or three parts; or finally as larger or smaller islands, heaps of cells of the serotina, detached from the cones and thus lost from the fatal placenta. Hence Winkler and KoHiker claim that the serotina contributes to the formation of the foetal placenta by furnish ing an envelope to the villosities. This is the sub-chorional layer of Wink lei.
In the course of the fifth month there appear in the serotina the mul tinucleated or giant cells, which are found in innumerable quantities in the maternal placenta at term. They develop and multiply with in credible rapidity. At first they appear among the most internal muscu lar fasciculi of the uterine parieties, especially in the neighborhood of the large vessels and in the deeper layers of the serotina. They soon gain the compact layer, and are found scattered irregularly among the other cells. (See Figs. 134 and 137.) The vessels, as at the fourth month, are composed of arteries, the walls of which are thinner in the neighborhood of the villosities, and which dilate as they open directly into the intervillous furrows. Thus the blood passes from the vessels into an immense reservoir of communicating lacuna, and is returned by the marginal vein and the veins of the sero tina (See Figs. 135 and 136.)
The marginal vein collects the blood from the lateral parts of the pla centa. It is a large vessel placed at the edge of the placenta, in the space that separates the serotina from the chorion. Its internal face presents several large openings, which communicate with the intervillous spaces..
The partitions separating these openings are formed by the villosities themselves, or by the above-described heaps of serotinal cells. Beyond this the venous walls approach one another, communicating frequently by short branches.
Thus the placental circulation is amply provided for. The arteries pour their contents into the blood-spaces; the villosities are plunged in it and bathed by it; and when these have carried off its oxygen, the blood itself passes into the marginal sinus and thence into the veins that ramify at the placental insertion.
A single thin layer of villous tissue, together with an epithelial coat, are all that separate the umbilical vessels in the villi from the blood of the mother; hence the rapid exchange of material between mother and child.
From the sixth to the seventh month there are no changes of any im portance.
At eight months an important phenomenon, to which Friedlander has called attention, occurs. Spontaneous thrombosis takes place in a certain number of veins about the placental insertion, caused by their tion by the giant cells of the serotina. Situated at first outside and along the veins, they insinuate themselves into the tissues of the vessels, and, once in the interior, cause coagulation of the blood by their presence. (Fig. 137.) At the Ow of Pregnancy (Figs. 138, 139).—There thus exists in the placenta a normal and constant venous stasis. But, if in consequence of too frequent thrombosis, or of an accident to the placental circulation,. this venous stasis exceeds its normal limits, may it not produce those sanguineous extravasations and fibrinous bodies which we encounter so often ? Might it not compromise the life of the fcetus? Can we not find in it the determining cause of the first uterine contractions and the com mencement of delivery? In fact, Brown-Siquard has shown that the ir ritability of the uterus increases as pregnancy goes on, and that the presence of carbonic acid gas in the maternal blood suffices to excite uterine contractions.
If this last explanation is correct (though, as Kehrer remarks, we must wait for experiments rr ore precise than those of Brown-Siquard), we will find in the venous hyperaemia of the placenta at the close of pregnancy a very important explanation of the appearance of the first pains.
We shall see, when we study the functions of the fo3tus, the important role played by the placenta.