The placenta may be inserted upon any part of the uterine cavity, although it is most usually near the fundus where the ovum must enter the womb. If, as is sometimes the case, it is attached at the lower part, over the orifice of the womb, it causes unavoidable hemorrhage in the later months of pregnancy.
The umbilical cord, funis, or naval string, is the connecting link between the child and mother. It commences when the external lamina of the blastoderm with the alantois are so changed as to form a mere cord upon which the two umbilical arteries ramify, and when all these have an enveloping sheath from the amnios. It may be discerned in this state at the end of the first month; at that time the fœtal intestines may be seen to protrude beyond the umbilicus into the amnionic sheath, but the cord is then cylindrical and very small. There are progressive changes, the cord becomes simplified, the canal of the amnionic sheath is obliterated gradually at its extremity, and as the effacement proceeds towards the umbilicus the intestine is pressed back so that no hernia remains.
There are two arteries in the cord; these arise from the abdominal aorta in the fœtus; they go by a flexed and tortuous course to the placenta, where they ramify and are distributed. There is only one vein which returns the blood from the placenta; there the radicles coalesce to form the branches; these unite to form the umbilical vein. This is not as flexuous as the arteries, which, being longer, wind around the one venous trunk. After the third month these may be plainly seen in the sheath imbedded in what has been called Wharton’s gellatine.
Ordinarily the cord lies free and loose in the cavity of the amnion, but occasionally owing to the movements of the child it may be coiled around the child’s neck, be tied in knots, or it may escape below the head so as to prolapse during labor.
The length of the cord varies; it is very rarely less than eight inches, and it is sometimes six or eight feet long.
After the birth of the child, the pulsation in the cord ceases within about fifteen minutes. After the cord is cut that portion that is attached to the umbilicus dies and usually falls off about the fifth day.
The blood of the fœtus is ærated or undergoes a change in the placenta analagous to the change that our blood undergoes in the lungs.
CHAPTER VIII.
THE FŒTUS.
The embryo first begins to be distinct about the third week; is then about two lines long, weighing one to two grains; is surrounded by an amnion which lies loosely about it, and obviously proceeds from the abdominal laminæ; it presents cerebral vesicles; there is the appearance of an eye, several arteries are seen though not distinctly formed; the abdominal cavity is open for a considerable extent in front.
About the fifth week the embryo becomes more consistent; the head is disproportionately large; rudimentary eyes are indicated by two black spots; the abdomen is nearly closed, though at the umbilical aperture a loop of intestine escapes; the abdominal members are present, and the cord exists in a rudimentary condition; the embryo is nearly two-thirds of an inch long and weighs about fifteen grains.