360 HUMAN UTERUS AND FETAL APPENDAGES. 



is loosened from the uterine wall and expelled, together with the fetal envelopes 

 and portions of the decidual membranes (uterine mucosa) of the mother; the parts 

 thus thrown off secondarily constitute the so-called after-birth of obstetricians. 



The placenta at full term, as thus obtained by natural expulsion, is a moist 

 mass, containing a great deal of blood, spongy in texture, about 7 inches in 

 diameter, but very variable in size, being roughly proportionate to the bulk of 

 the child; usually oval, sometimes round, but not infrequently irregular in shape. 

 One surface is smooth and covered by a pellucid membrane (the amnion), and 

 reddish gray in color; to this surface the umbilical cord is attached, and it shows 

 the arteries and veins branching out irregularly from the cord over the surface of 

 the placenta (Fig. 240). The opposite surface is rough, lacerated, and usually 

 covered irregularly with more or less blood, which is often dark and clotted. 

 When the blood is removed, the surface is seen to be crossed by a system of 

 grooves which divide the placental tissue into irregular areas, each perhaps an inch 

 or so in diameter; these areas are called cotyledons. The placenta is about 25 or 

 30 mm. thick, but thins out rapidly at the edges, and its tissue passes over from 

 the margin of the placenta. 



When in situ, the placenta is fastened to the walls of the uferus by its rough 

 or cotyledonary surface; its smooth, amniotic surface faces the cavity in which the 

 fetus lies. 



A more detailed examination of the gross appearance of a placenta discharged 

 at term leads to the following additional observations: The color is a reddish or 

 purplish gray, varying in tint according to the condition of the blood, and mottled 

 between the divaricating blood-vessels by patches and networks of pale yellowish 

 or flesh color. The light pattern is produced by the tissue of the villi shining 

 through the membrane of the chorion. These appearances are less distinct when 

 the placenta, as is usually the case, is covered by the thin amnion. The amnion, 

 however, is very easily detached as far as the insertion of the umbilical cord, to 

 the end of which it is firmly attached, but it cannot be traced farther because on 

 the cord itself there is no amnion. The blood-vessels run out in all directions 

 from the end of the cord; each vessel produces a ridge upon the placental surface, 

 so that its course is readily followed. The arteries and veins are more easily 

 distinguished after double injection, as is shown in figure 240. 



The two kinds of vessels do not run together; the arteries lie near the surface, 

 just above the veins; the arteries fork repeatedly, until they are represented only 

 by small branches and fine vessels; some of the small branches disappear quite 

 suddenly by dipping down into the deeper-lying tissue in order to pass into the 

 villi. The veins (Fig. 240) are considerably larger than the arteries; they branch in 

 a similar manner, but some of the trunks disappear from the surface more abruptly 

 than is the case with the arteries. There is the greatest possible variability in 

 the vessels of the placenta; one never sees two placentae with vessels alike. 



The insertion of the cord is always eccentric; the degree of eccentricity is 



