THE HUMAN PLACENTA. . 339 



The amnion, however, is very easily detached as far as the insertion of the um- 

 bilical 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 196. 



The two kinds of vessels do not run together ; the arteries lie near the sur- 

 face, 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. 196) 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; I have never seen two placentae with 

 vessels alike. So far as I have observed, the insertion of the cord is always ob- 

 viously eccentric ; the degree of eccentricity is variable and is easily seen to be 

 related to the distribution of the vessels. 



The insertion of the cord may even be entirely outside the placenta, which 

 yet may otherwise be normally developed. Such insertions are called velamen- 

 tous. The usual type is shown in figure 196. The arteries come down together 

 from the cord; they usually, but not always, anastomose by a short transverse 

 vessel, which lies about half an inch above the surface of the placenta; it could 

 not be shown in the figure. Very rarely, if ever, are there any arterial or venous 

 anastomoses on the surface of the placenta. The arteries there spread out in a 

 manner which may be described as roughly symmetrical. The veins partially 

 follow the course of the arteries. When the cord is inserted near the margin, the 

 symmetry of the placental vessels is greater, when the insertion is near the cen- 

 ter, the symmetry is less, than in the figure. 



The reverse or uterine surface of the placenta is rough and divided into 

 numerous rounded oval or angular portions termed lobes or cotyledons, as stated 

 above. These vary from half an inch to an inch and a half in diameter. The 

 whole of this surface consists of a thin, soft, somewhat leathery investment by the 

 decidual membrane, which dips down in various parts to form the grooves that 

 separate the cotyledons from each other. This layer is a portion of the decidua 

 serotina, which, as long as the parts are in situ, constitutes the boundary between 

 the placenta and the muscular substance of the uterus, but which at the time of 

 labor becomes split asunder, so that while a portion is carried off along with the 

 placenta and constitutes its external membrane, the rest remains attached to the 

 inner surface of the uterus. If a placenta is cut through, it is found to consist of 



