344 HUMAN UTERUS AND FETAL APPENDAGES. 



of blood-vessels, many of which are large, irregular sinuses. The uterine walls 

 consist of an outer muscular layer, and an inner decidual layer; the latter takes up 

 nearly half the thickness of the wall, and is known as the decidua vera. Com- 

 parison with the seventh month uterus shows that the proportion of the layers 

 changes, because during gestation the muscular layer increases and the decidual 

 layer diminishes in thickness. The inner bag, when opened, shows the large 

 cavity in which .the embryo lies floating in amniotic fluid. The bag is formed by 

 three very distinct membranes, of which the outermost, the decidua reflexa, is 

 opaque and the thickest; the two inner ones are thin and transparent; the inner- 

 most is the delicate amnion; the middle membrane is the chorion, and is quite 

 distinct from -both the amnion and reflexa; to the latter it is connected by a 

 number of small branching villi scattered at some distance from one another over 

 the surface; the villi adhere firmly to the reflexa by their tips. The embryo 

 (Fig. 109) resembles a child in its general appearance; the length of the head 

 and rump together is nearly 8 cm., and the head is approximately equal in bulk 

 to the rump. The umbilical cord is from 5 to 7 mm. in diameter and usually 

 about 12 cm. long. From its distal end the blood-vessels spread out over the 

 placental area, and around the edge of the area rises the decidua reflexa, which 

 does not extend on to the placenta. Floating in the amniotic fluid is a pear- 

 shaped vesicle, the yolk-sac, which is about 7 mm. in diameter; it has a fine 

 network of blood-vessels upon its surface, and is connected at its pointed end with 

 a long, slender pedicle, the yolk-stalk, which runs to the placental end of the 

 umbilical cord, there enters the cord itself, and runs through its entire length to its 

 attachment to one of the coils of the intestine of the embryo.* Over the whole 

 of the placental area the chorion gives off large villous trunks, each of which has 

 numerous branches, with ramifications of the fetal vessels; the villi fill a space 

 about i cm. wide between the membrane of the chorion frondosum and the 

 surface of the uterine decidua serotina, to which the tips of some of the villi 

 are attached. With care the villi may be separated from the decidua, which is 

 seen, when it is thus uncovered, to be cavernous;. the caverns are rounded in form 

 and part of them may be followed, on the one hand, until they connect with the 

 blood sinuses of the uterus, and, on the other, until they open into the intervillous 

 spaces, which therefore receive a direct supply of blood from the mother. 



The principal difference to be noted between the uterus before and that after 

 the fifth month in the relations of parts is the presence or absence of the decidua 

 reflexa as a distinct membrane. Duping the fourth month the reflexa stretches 

 as the membranes expand, and becomes thinner and thinner until by the end of 

 the fourth month it is as delicate and transparent as the chorion and lies close 

 against the decidua vera. 



* At this stage a large part of the yolk-stalk within the umbilical cord has degenerated and usually disap- 

 peared by resorption. 



