As the embryo develops and the liquor amnii increases, the amnion is for is forced to recede more and more from the foetus, o course it must eventually be forced to join the outer membrane, or chorion, and thus form a covering to the umbilical cord all the way from the navel of the foetus to the outer membrane or chorion into which its maternal end is inserted; and all the parts thus enclosed constitute what is called the umbilical cord. Consequently the whole abdominal cavity must be in connection with the canal represented by this cord, that the foetal appendages may communicate with it through the opening thus prepared for the. It is in this manner that the pedicle of the umbilical vesicle becomes united to the ileo-caecal fold of the intestine whilst the allantois connects with the rectum by the the intervention of the urachus. As the development of the ovum advances, the amnion grows more and more like serous membrane in firmness and consistency, although it never become enclosed, for does it at any period possess and vessels.
The liquor amnii itself varies in appearance with the time at which it is examined. At the earliest period, it is clear, transparent and limpid; at later periods it become thicker, more opaque, sometimes flaky, green, yellow, or of almost any other color, according to certain states of the foetus itself, which it derives from corresponding conditions of the mother. Its odor varies also; sometimes it is like that of spermatic fluid, with saline taste. the proportionate quantity, of he liquor amnii also varies according to the time; in the earlier periods it is much greater than the weight of the foetus. At the middle of the term the relation may be about equal; and after that time the liquid lessens in proportion, as the foetus increases. So that at full term the quantity of water is usually about a pound and a half, sometimes more and sometimes much less.
The analysis of his fluid usually shows 98.8 of water; albumen, hydrochlorate of soda, phosphate of lime and lime, 1.2 parts in a hundred.
The formation of the embryo involves the formation of the amnion as a matter of course, as a reservoir for the reception of effete matter from the foetus. the amnion also affords protection to the foetus by surrounding its body with aqueous shield, impervious to blows and shocks, since the embryo floats freely in its centre. The foetus, thus floating in the liquor amnii, is at liberty to accommodate itself to all the movements and varying positions of the mother, and to yield, with the least possible disturbance, to all the influences of her mental and moral states. The accoucheur finds in the ballottement, practised by the aid the period of parturition is able to correct any malpositions with greater ease before than after waters have been discharged.
The chorion, it is now very evident, consists in the later stages of pregnancy, first, f an outer membrane, the original vitelline membrane; next to this lies the blastodermic membrane; the former being called the endochorion, the latter the endochorion; within this last lies the amniotic membrane so that at the time of labor we are often conscious of rupturing these three distinct membranes.
THE PLACENTA, AFTER-BIRTH, SECUNDINES. In order to obtain a proper idea of the placenta, it is necessary to commence its description at a very commencement of its formation. It will be recollected that the allantois shoots from the embryo at a very early period of its existence and very soon take root in the villi of the chorion. Examination and experiments that this process is adapted to furnish nourishment to the embryo by means of arteries, he one vein, passing front the embryo to the mother and back. The arteries carrying the impoverished arterial blood from the foetus towards the mother, seeking aeration and nourishment in the mother. the vein brings it back, purified and laden with all that the embryo needs for its growth and nourishment. In this first circuit there is made the beginning of the placenta, or the placenta itself so far. the next. Circuit adds to its size the larger demand mace by the increasing embryo and so its goes on, constantly enlarging circuit after circuit, until the accomplishment of the full time.
The placental mass at its full size, is about six inches in diameter; three -fourths of an inch thick in its centre, tapering to a thin edge at tits circumference. In different persons the placenta will often be found to very in form and size but the above are about the average dimensions. The cord is usually attacked in the centre; but it is
Fig.52 Placenta with five separate Cotyledons.
A. Chorion. B. Amnion. C. The cord. D. Separate Cotyledons.
sometimes found attached to one edge, constituting what is termed a battle-door placenta. The placenta presents an internal or foetal surface, and and external, maternal, or uterine surface and a border. At full term the internal surface is found covered by the chorion and the amnion; while numerous ramifications of he umbilical vein and arteries are seen (as in the cut) to coverage from the circumference to the centre to form the umbilical cord. This convergence commences with the very first circuit, after the allantoid vesicle has taken root in the villi; and it continues becoming more and more largely developed till the completion of the full term. The foetal surface is smooth, concave and transparent, showing these ramifications most beautifully at full term. The maternal surface is convex, rough, and subdivided into a variable number of lobes, held together by an albuminous tissue which is so easily torn off that it is very difficult to retain it is separating the placenta. These lobes, or cotyledons, are the original villi of he chorion, very much enlarged and compacted together; although even these are also very easily ruptured and separated into loose disorganized masses.
The villi have been described as compacted together; this results from their having grown into the mucous membrane of the uterus, between that organ and the ovum. At the same time the mucous membrane has also grown into the villi and the chorion; so that there is a mutual growth of one in to the other. each supplying its own share in the formation of the placenta: the villi and their growth
Fig.53. Fig.54.
Fig.53. The internal or foetal surface of the placenta.
Fig.54. The external or uterine surface of the placenta.
forming he foetal side, and the mucous membrane and its growth forming the maternal or uterine side of the placenta. Thus, in the very first circuit of the foetal blood in its vessels on the inner side of these villi, it becomes aerated and supplied with all things needful from the maternal blood on the outer side of these villi. In this way the process on, hour after hour, day after day, the demand and the supply alike increasing, till large rivers of blood are formed on each side; their banks, however, never breaking, while the foetal blood constantly communications, but never commingle, s with the maternal blood. This is proved by injecting the vessels on the foetal side; not a particle, even of the finest material, is ever found to pass beyond of the foetal side, while on the other hand, it is found equally impossible to cause a particle of the finest injection to pass from the maternal to the foetal side.
When the placenta is peeled off from the uterus, large open mouths of blood-vessels are seen, which show how the blood passes from the uterus, through the mucous membrane as its medium, towards the foetus. In fact, blood-vessels, arteries, and nerves come from the mother, through the uterus and its mucous membrane, to meet the demands of the foetus, in a manner similar to that in which we see the ramifications of the cord diverging from the foetus, upon the foetal surface of the placenta, to make its demands upon the mother. These opposite streams meet and embrace each other, enterwine with each other, mutually giving to and receiving from each other, in each successive moment from the first insertion of the allantois into the villi of the chorion till the accomplishment of the full time.
Thus the structure of the placenta is seen to be strictly vascular, being entirely composed of arteries and veins; those of the foetus intertwining with those of the mother; the chorion and its villi constituting the septum between the two, and through which fluids can pass only by endosmosis. After the birth of the child all comes away in a mass; the chorion and the amnion constituting the membranes, which are sometimes called also the secundines. The placenta is usually attached to the fundus of the uterus; but the attachment may be at the side, or upon the lowest segment, from the ovule sinking to that spot at the first.
THE UMBILICAL CORD. This organ is the flexible trunk by which the foetus in utero is connected with the mother; it unites the abdomen of the child with the placenta just described as adherent to the walls of the uterus. The product of a somewhat advanced state of development, it is not found in the early weeks of pregnancy. The umbilical cord takes its origin in the embryo, in the form of the allantoid vesicle. And as soon as this vesicle has taken root in the villi of the chorion it is found to consist of two arteries arising from the bifurcation of the abdominal aorta in the embryo, and of one accompanying vein, which arises from the vena cava ascendens and the hepatic portal vein. The arteries carry the embryonic blood away, to be replenished nourished by that of the parent. This is accomplished by its dipping into the villi of the chorion. From them the blood is returned by the vein to the embryo;l and thus is established the living connection of the foetus with the mother.
