Placenta

in anatomy, (from mAzxse, a cake, so called from its resemblance to a cake.) ‘The placenta is a circular, flat, vascular, and apparently fleshy substance, different in its diameter in different subjects; but usually, extending about six inches, or upwards,.over about one-fourth part of the outside of the ovum in pregnant women. Itis more than one inch in thickne in the middle, and becomes gradually thinner towards the circumference from which the membranes are continued. The placenta ig the principal medium by which the communication between the parent and child is’ pre= — served; but though all have allowed the ims poriance of the oflice which it performs, there has been a variety of opinions on the nature of — that office, and of the manner in which it is” executed, ‘That surface of the placenta which is attached to the uterus by the intervention of the connecting membrane is lobulated and convex; but the other, which is covered with the amnion and chorion, is concave and smooth, except the little eminences made by the blood ve els. It is seldom found atttached to the same part of the uterus in two succe ive births 5 and, though it most frequently adheres to the anterior part, it is oecasionally fixed to any — other, even to the os uteri, in which: state it becomes a cause of a dangerous hemorrhage at the time of parturition. The placenta is composed of arteries and veins, with a mixture of pulpy or cellular substance, Of these there are two orders, very curiously interwoven with each other. ‘The first is a continuation of those fram the fun is, which ramify on the internal surface of the placenta, the arteries running over the veins, which is a circumstance peculiar to the placenta; and then, sinking 1nto its substance, anastomose and divide into innttmerable small branches. The -second order proceeds from the uterus; and, these ramify ina similar manner with those from the fun is, as appears when a placenta is injected from those of the parent, The veins,: in their ramifications, accompany the arteries as. in other parts. There have been many different opinions with respect to the manner in which the blood circulates between the parent and child, during its continuance in the uterus. For a long time it was believed that the inter. course between them was uninterrupted, and that the blood propelled by the powers of the parent pervaded, by a continuance of the same | force, the vascular system of the fetus; but repeated attempts having been made without succe to inject the whole placenta, fun is and foetus, from the’ve els of the parent, or any part of the uterus, from the ve els of the fun is, it is now generally allowed, that the two systems of ve els in the placenta, one of which may be called maternal, the other feetal, are distinct. Itis also admitted, that the blood of the foetus is, with regard to its formation, increase, and circulation, unconnected with and totally independent of the parent: except that the matter by which the blood of the fetus is formed must be derived from the parent. is thought that which has probably undergone some preparatory changes in its pa age through the uterus, is conducted by the uterine or.maternal arteries of the placenta to some cells or small cavities, in which it is deposited;.and _ that some part of it; or something secreted from it, is absorbed by the foetal veins of the placenta, and by them conveyed to the fetus for its nutriment. When the blood which — circulates in the fetus requires any alteration in its qualities, or when it has gone through the course of the circulation, it is carried by ve arteries of the fun is to the placenta, in the cells of which it is deposited, and then absorbed by the maternal veins of the placenta, and ' conducted to the uterus, whence it may enter the conimon circulation of the parent. Thus it appears, according to the opinion of Harvey, that the placenta performs the office of a gland, conveying air, or secreting the nutritious juices from the blood brought from the parent by the arteries of the uterus, and carried to the fetus by the veins of the fun is, in a manner probably not unlike to that in which milk is secreted and absorbed frou: the breasts. The veins in the placenta are mentioned as the absorbents, because no lymphatic ve els have yet been found in the placenta or fun is; nor are there any nerves in these parts; so that the only communications hitherto discovered between » the parent and child, is by the sanguineous system. The proofs of the manner in which the plood circulates be:ween the parent and child are chiefly drawn from observations made upon the fun is. When it was supposed that the child was supplied with blood in a direct stream from the parent, it was a erted that, on the division of the fun is, if that part next to the placenta was not secured by a ligature, the parent would be brought into extreme danger by the hemorrhage which must nece arily follow. But this opinion, which laid the foundation of several peculiarities in the man a gem i ent of the fun is and placenta, is proved noi to be true: for, if ihe fun is be compre ed immediately after the birth of the child, and whilst the circulation in it is going on, the arteries between the part compre ed and the child throb violently, but those between the compre ion and the placenta have no pulsation; but the vein between the part compre ed and the placenta swells, and that part next to the fetus becomes flaccid. But if, under the same circumstances, the fun is be divided, and that part next the child be not secured, the child would be in danger of losing its life by the hemorrhage; yet the mother would suffer no inconvenience if the ether part was neglected. It is moreover proved, that a woman may die of an hemorrhage occasioned by a separation of the placenta, and the child be neverthele born, after her death, in perfect health. But if the placenta be injured, without separation, either by the rupture of the ve els which pa upon its inner surface, or in any other way, the child being deprived of its proper blood, would perish, yet the parent might escape without injury. See also SECUNDINEs. 477