A System of Midwifery

by Edward Rigby

On examining these vessels in an injected uterus to which the placenta is attached, we shall therefore find that all traces of a regular canal or tube are suddenly lost upon their entering the placenta; each vessel (whether artery or vein) abruptly terminating in a spongy cellular tissue. If a blow-pipe be introduced into a piece of sponge, we shall have a very simple but correct illustration of the manner in which the uterine blood circulates through the placenta. The cell into which each vessel immediately opens is usually much larger than the rest, so that when the cellular structure of the placenta is filled with wax, a number of irregular nodules[22] are found continuous with these vessels and passing into an infinity of minute granules, which are merely so many casts of smaller cells. That this cellular tissue pervades the whole mass of the placenta, and communicates freely with the uterine vessels by which it is filled with blood, is proved by repeating a very simple experiment of Dr. Hunter, viz. “if a blow-pipe be thrust into the substance of the placenta any where, the air which is blown into the cellular part opens, and rushes out readily by, the open mouths both of the arteries and veins.” (Hunter, op. cit. p. 46.) That it also envelopes the umbilical vessels of the cord is shown by the fact, that if a pipe be inserted beneath the outer covering of the cord near to its insertion into the placenta, we shall be able to “fill the whole placenta uniformly in its cellular part, and likewise all the venous system of the uterus and decidua, as readily and fully as if we had fixed the pipe in the spermatic or hypogastric vein; so ready a passage is there reciprocally between the cells of the placenta and the uterine vessels.” (Ibid. p. 47.)

The maternal portion of the placenta therefore consists of a spongy cellular tissue, which is filled by the uterine vessels, and also of those trunks which pass through the decidua, and which form the communication between these vessels and the placental cells.

Fœtal surface of the placenta.

The fœtal surface of the placenta is smooth and glossy, being covered by the amnion and chorion; it is much harder than the uterine surface, and is streaked over by the larger branches of the umbilical vein and arteries, which radiate irregularly from the point where the cord is inserted; and which pass beneath the amnion, and between the two layers of which the chorion is composed, to which they are intimately connected. These vessels supply the various lobuli of which the placenta is composed, so that each lobulus receives at least one of these branches; for, although the umbilical cord consists of two arteries and one vein, this arrangement does not continue into the body of the placenta. “Every branch of an artery,” as Dr. Hunter observes, “is attended with a branch of a vein: these cling to one another, and frequently in the substance of the placenta entwine round one another, as in the navel string.” (Ibid. p. 40.) Each cotyledon receives its own vessels, so that the vessels of one cotyledon have no direct communication with those of the adjacent ones, as proved by Wrisberg’s examinations; for if we inject the vessel or vessels of one of these lobuli, the injection will not pass into those of the others. When the vessels have reached the cotyledons, they are divided and subdivided ad infinitum; they are connected together by a fine cellular membrane, which may be very easily removed by maceration, and then they may be seen ramifying in the most beautiful and delicate manner possible; the main branches having no communication or anastomosis with each other.

The umbilical arteries anastomose freely with each other upon the fœtal surface of the placenta, before dividing into the branches above-mentioned; hence, if an injection be thrown into one umbilical artery it will return almost immediately by the other; but if this be tied also, the injection, after a time, will return by the umbilical vein, but not until all the vessels of the placenta have been filled, proving that there is a free passage of blood from the arteries into the veins.

From these remarks, founded chiefly on the admirable observations of the Hunters, and repeated examinations of the placenta, which we have made with the greatest care and impartiality, it may be stated with confidence, that the placenta consists of two portions—a maternal and a fœtal. The maternal portion consists, as we have before observed, of a spongy cellular tissue; and also of those trunks which pass through the decidua, and which form the communication between the uterine vessels and the placental cells. The fœtal part is formed by the ramifications of the umbilical vessels: “that each of those parts has its peculiar system of arteries and veins, and its peculiar circulation, receiving blood by its arteries, and returning it by its veins; that the circulation through these parts of the placenta differs in the following manner: in the umbilical portion the arteries terminate in the veins by a continuity of canal; whereas, in the uterine portion there are intermediate cells into which the arteries terminate, and from which the veins begin.” (Hunter, op. cit. p. 48.)

Although various observations and anatomical injections show that to a certain degree, there is a communication between the uterus and the placenta, inasmuch as the blood of the former is received into the sinuses or cells of the latter, we possess no proof that the blood can pass from these sinuses into the umbilical vessels: on the contrary, every thing combines to prove that the circulation of the fœtus is altogether independent of that of the mother. We know from daily experience that in labour at the full term of pregnancy, the placenta is easily expelled from the uterus: that, upon examining the surface which had been attached to the uterus we find no laceration, and that a discharge of more or less blood takes place for some days afterwards. We know, also, that when the placenta becomes detached from the uterus during the progress of gestation, it is followed by a considerable hemorrhage, which greatly endangers the life of the mother. These facts prove that there is a circulation of uterine blood in the placenta, which is destroyed upon its being separated from the uterus. That this uterine circulation in the placenta is unconnected with the circulation of fœtal vessels in the placenta is proved by the fact first pointed out by Wrisberg, viz. that, where the mother has died from loss of blood, and the maternal vessels therefore drained of their contents, those of the fœtus have been full of blood. Still farther to illustrate this fact, he killed several cows big with calf, by a large wound through the heart or great vessels, so as to ensure the most profuse and sudden loss of blood possible, and never found that the vessels of the calf were deprived of blood, although those of the mother were perfectly empty; moreover, no anatomist has ever yet succeeded in making injections pass from the fœtal into the uterine vessels, or vice versâ. Lobstein has mentioned a mode of illustrating this fact (Essai sur la Nutrition du Fœtus,) which is both simple and striking. Upon examining the uterine surface of a placenta which has been expelled at the full term, it presents the appearance of a spongy mass gorged with blood, which may be removed by washing or maceration, and if a placenta thus prepared be injected, the fluids will pass with the greatest facility from the umbilical arteries into the umbilical vein, but not one drop into its cellular structure; it is evident, therefore, that the blood which had filled the intervals between the vessels, and which had been removed by washing and maceration, could not have belonged to the fœtus, but must have come from the mother; for if any of the vessels had been ruptured the injection would not have succeeded.

In concluding these observations upon the placenta, we may briefly state, that there is the same relation between the umbilical vessels and the maternal blood, which fills the placental cells, as there is between the branches of the pulmonary artery, and the air which fills the bronchial cell.[23]

Umbilical cord. The umbilical cord, funis, or navel string, is a vascular rope extending between the fœtus and placenta, by which they are connected together. It usually arises, as we have before observed, from about the middle of the placenta, and terminates at the umbilical ring of the fœtus; it consists of two umbilical arteries and one umbilical vein; the former conveying the blood from the common iliac arteries of the fœtus to the cotyledons of the placenta; the latter formed by the union of the collected umbilical veins, on the inner surface of the placenta, and returning this blood to the fœtus. In the early periods of pregnancy it also consists of the duct and vessels of the vesicula umbilicalis, the urachus, and more or less of the intestinal canal. The umbilical cord does not present the same form or appearance at every period of gestation; the younger the embryo, the shorter and thicker is the cord; in fact, there are no traces whatever of a cord at first, the embryo adhering, by its lower or caudal extremity, directly to the membranes. By the fifth or sixth week it becomes visible; at this early period the vessels of which it is composed pass from the fœtus in a straight direction, but as pregnancy advances they become more or less spiral, winding round each other, and usually from left to right: according to Meckel, they take the opposite direction much less frequently, viz. in the proportion of one to nine.