Fig. 209.

period in the villosities of the chorion. The villosities of the chorion are now, therefore, hardly anything more than ramified and tortuous vascular loops; the remaining substance of the villi having been atrophied and absorbed in the excessive growth of the blood vessels. (Fig. 209.) The uterine follicles have at the same time lost all trace of their original structure, and have become mere vascular sinuses, into which the tufted foetal blood vessels are received, as the villosities of the chorion were at first received into the uterine follicles..

Finally, the walls of the foetal bloodvessels having come into close contact with the walls of the maternal sinuses, the two become adherent and fuse together; so that a time at last arrives, when we can no longer separate the foetal vessels, in the substance of the placenta, from the maternal sinuses without lacerating either the one or the other, owing to the secondary adhesion which has taken place between them.

The placenta, therefore, when perfectly formed, has the structure which is shown in the accompanying diagram (Fig. 210), repre

Vertical section of PLACENTA, showing arrangement of maternal and fatal vessels. rion. b, b. Decidua. c, c, c, c. Orifices of uterine sinuses.

a, a. Cho

senting a vertical section of the organ through its entire thickness, At a, a, is seen the chorion, receiving the umbilical vessels from the body of the foetus through the umbilical cord, and sending out its compound and ramified vascular tufts into the substance of the placenta. At bb, is the attached surface of the decidua, or uterine mucous membrane; and at c, c, c, c, are the orifices of uterine vessels which penetrate it from below. These vessels enter the placenta in an extremely oblique direction, though they are represented in the diagram, for the sake of distinctness, as nearly perpendicular. When they have once penetrated, however, the lower portion of the decidua, they immediately dilate into the placental sinuses (represented, in the diagram, in black), which extend through the whole thickness of the organ, closely embracing all the ramifications of the foetal tufts. It will be seen, therefore, that the placenta, arrived at this stage of completion, is composed essentially of nothing but blood vessels. No other tissues enter into its structure, for all those which it originally contained have disappeared, excepting the blood vessels of the foetus, entangled with and adherent to the blood vessels of the mother.

There is, however, no direct communication between the foetal and maternal vessels. The blood of the foetus is always separated from the blood of the mother by a membrane which has resulted from the successive union and fusion of four different membranes, viz., first, the membrane of the foetal villus; secondly, that of the uterine follicle; thirdly, the wall of the foetal blood vessel; and, fourthly, the wall of the uterine sinus. The single membrane, however, into which these four finally coalesce, is extremely thin, as we have seen, and of enormous extent, owing to the extremely abundant branching and subdivision of the foetal tufts. These tufts, accordingly, in which the blood of the foetus circulates, are bathed everywhere, in the placental sinuses, with the blood of the mother; and the processes of endosmosis and exosmosis, of exhalation and absorption, go on between the two with the greatest possible activity.

It is very easy to demonstrate the arrangement of the foetal tufts in the human placenta. They can be readily seen by the naked eye, and may be easily traced from their attachment at the under surface of the chorion to their termination near the uterine surface of the placenta. The anatomical disposition of the pla cental sinuses, however, is much more difficult of examination. During life, and while the placenta is still attached to the uterus,

they are filled, of course, with the blood of the mother and occupy fully one-half the entire mass of the placenta. But when the pla centa is detached, the maternal vessels belonging to it are torn off at their necks (Fig. 210, c, c, c, c), and the sinuses, being then emptied of blood by the compression to which the placenta is subjected, are apparently obliterated; and the foetal tufts, falling together and lying in contact with each other, appear to constitute the whole of the placental mass. The existence of the placental sinuses, however, and their true extent, may be satisfactorily demonstrated in the following manner.

If we take the uterus of a woman who has died undelivered at the full term or thereabout, and open it in such a way as to avoid wounding the placenta, this organ will be seen remaining attached to the uterine surface, with all its vascular connections complete. Let the fœtus now be removed by dividing the umbilical cord, and the uterus, with the placenta attached, placed under water, with its internal surface uppermost. If the end of a blowpipe be now introduced into one of the divided vessels of the uterine walls, and air forced in by gentle insufflation, we can easily inflate, first, the venous sinuses of the uterus itself, and next, the deeper portions of the placenta; and lastly, the bubbles of air insinuate themselves everywhere between the foetal tufts, and appear in the most superficial portions of the placenta, immediately underneath the transparent chorion (a a, Fig. 210); thus showing that the placental sinuses, which freely communicate with the uterine vessels, really occupy the entire thickness of the placenta, and are equally extensive with the tufts of the chorion. We have verified this fact in the above manner, on four different occasions, and in the presence of Prof. C. R. Gilman, Dr. Geo. T. Elliot, Dr. Henry B. Sands, Dr. T. G. Thomas, Dr. T. C. Finnell, and various other medical gentlemen of New York.

If the placenta be now detached and examined separately, it will be found to present upon its uterine surface a number of openings which are extremely oblique in their position, and which are accordingly bounded on one side by a very thin, projecting, crescentic edge. These are the orifices of the uterine vessels, passing into the placenta and torn off at their necks, as above described; and by carefully following them with the probe and scissors, they are found to lead at once into extensive empty cavities (the pla cental sinuses), situated between the foetal tufts. We have already shown that these cavities are filled during life with the maternal

blood; and there is every reason to believe that before delivery, and while the circulation is going on, the placenta is at least twice as large as after it has been detached and expelled from the uterus.

The placenta, accordingly, is a double organ, formed partly by the chorion and partly by the decidua; and consisting of maternal and foetal blood vessels, inextricably entangled and united with each other.

The part which this organ takes in the development of the foetus is an exceedingly important one. From the date of its formation, at about the beginning of the fourth month, it constitutes the only channel through which nourishment is conveyed from the mother to the fœtus. The nutritious materials, which circulate in abun dance in the blood of the maternal sinuses, pass through the intervening membrane by endosmosis, and enter the blood of the foetus. The healthy or injurious regimen, to which the mother is subjected, will accordingly exert an almost immediate influence upon the child. Even medicinal substances, taken by the mother and absorbed into her circulation, may readily transude through the pla cental vessels; and they have been known in this way to exert a specific effect upon the foetal organization.

The placenta is, furthermore, an organ of exhalation as well as of absorption. The excrementitious substances, produced in the circulation of the foetus, are undoubtedly in great measure disposed of by transudation through the walls of the placental vessels, to be afterward discharged by the excretory organs of the mother. The system of the mother may therefore be affected in this manner by influences derived from the foetus. It has been remarked more than once, in the lower animals, that when the female has two successive litters of young by different males, the young of the second litter will sometimes bear marks resembling those of the first male. In these instances, the peculiar influence which produces the external mark must have been transmitted by the first male directly to the foetus, from the foetus to the mother, and from the mother to the foetus of the second litter.

It is also through the placental circulation that those disturbing effects are produced upon the nutrition of the foetus, which result from sudden shocks or injuries inflicted upon the mother. There is now little room for doubt that various deformities and deficiencies of the fœtus, conformably to the popular belief, do really originate, in certain cases, from nervous impressions, such as disgust, fear or anger, experienced by the mother. The mode in which these effects may

be produced is readily understood from what has been said above of the anatomy and functions of the placenta. We know very well how easily nervous impressions will disturb the circulation in the brain, the face, the lungs, &c.; and the uterine circulation is quite as readily influenced by similar causes, as physicians see every day in cases of amenorrhoea, menorrhagia, &c. If a nervous shock may excite premature contraction in the muscular fibres of the pregnant uterus and produce abortion, as not unfrequently happens, it is certainly capable of disturbing the course of the circulation through the same organ. But the foetal circulation is dependent, to a great extent, on the maternal. Since the two sets of vessels are so closely entwined in the placenta, and since the foetal blood has here much the same relation to the maternal, that the blood in the pulmonary capillaries has to the air in the air-vesicles, it will be liable to de rangement from similar causes. If the circulation of air through the pulmonary tubes and vesicles be suspended, that of the blood through the capillaries comes to an end also. In the same way, whatever disturbs or arrests the circulation through the vessels of the maternal uterus must necessarily be liable to interfere with that in the foetal capillaries forming part of the placenta. And lastly, as the nutrition of the foetus is provided for wholly by the placenta, it will of course suffer immediately from any such disturbance of the placental circulation. These effects may be manifested either in the general atrophy and death of the foetus; or, if the disturbing cause be slight, in the atrophy or imperfect development of particular parts; just as, in the adult, a morbid cause operating through the entire system, may be first or even exclusively manifested in some particular organ, which is more sensitive to its influence than other parts.

The placenta must accordingly be regarded as an organ which performs, during intra-uterine life, offices similar to those of the lungs and the intestine after birth. It absorbs nourishment, renovates the blood, and discharges by exhalation various excrementitious matters, which originate in the processes of foetal nutrition.