Chorion.— The same proportion of chorion as of the amnion, namely, about one third of the entire superficies, is appropriated to the placenta. This, however, is not, like the amnion, simply an apposed membrane. It enters into the composition of the organ, and gives strength to it (fig. 484. ch). It sustains and transmits the branches of the umbilical vessels (vf ), which adhere to, and ramify upon, its fcetal surface, between it and the amnion. This face of the chorion is united to the amnion by a thin and easily separable layer of soft pulpy tissue, constituting a portion of the tunica media of the ovum, while the reverse surface, which forms, as it were, the base or floor of the placenta, bears the numerous tufts or villi that make up the bulk of this organ.
Falai blood-vessels.— The blood-vessels of the placenta which belong to the (Cetus are branches of the two umbilical arteries, and of the single umbilical vein. These, as just stated, ramify in large trunks over a consider able portion of the fcetal or under surface of the placenta, before they penetrate the cho rion to gain the interior of the organ. When the fcetal vessels have been injected from the funis, their course upon this surface of the placenta is easily traced. Within the root of the cord, and at a distance of one inch from its insertion, the two umbilical arteries com municate together by a cross branch half an inch in length. Immediately on reaching the placental surface, each artery bifurcates, the branches passing off in opposite directions. A second bifurcation takes place, in the same manner, about half or three quarters of an inch from the first. And lastly, a third, at distances ranging from one to two and a half inches. Each of these dichotomous divisions is at first more or less abrupt and opposite, the vessels afterwards bending, and taking a slightly divergent or parallel course, or even somewhat approximating. After the third bifurcation, the vessels again divide and sub divide, but now at acute angles ; their extre mities become lost, when they are reduced to the size of a crow quill, by dipping down sud denly, and passing through the chorion, to enter the substance of the placenta at dis tances varying from an inch to an inch and a half from its border. A sinall branch, how ever, in continuation, often runs on nearly to the edge. Lateral branches, of the same size as the terminal subdivisions, also leave the main vessels in all parts of their course, and dip down into the placental substance.
The branches of the veins, about sixteen in number, which return the blood from the in terior of the placenta, emerge from its sub stance close to the points of entrance of the arteries, and take a less tortuous course than the latter. They, however, accompany these
vessels, but more in the form of radiating lines, which proceed towards the root of the funis, passing under the arteries, and ulti mately uniting in the single umbilical vein.
The varieties in the form of the placenta already noticed are apparently dependent upon certain modifications in the development and arrangement of these vessels, which are like wise very variable, although the same primary divisions are noticeable in all. In the circular placenta the root of the cord is inserted into, or near, the centre. In the oval form it is attached to the smaller extremity forming the placenta en raquette. In the renifornz and cordate placenta, the insertion is likewise more or less lateral. Lastly, when the vessels of the cord divide before arriving at the surface, they form the placenta en parasol.
Uterine surface.— The reverse or uterine surface of a placenta vvhich has been sepa rated from its attachment, as in natural labour, is rough, and is divided into numerous rounded oval or angular portions, termed lobes or co tyledons. These vary from half an inch to an inch and a half in diameter. The whole of this surface consists of a thin, soft, and somewhat leathery investment of deciduous membrane, which dips down in various parts to form the sulci that separate the cotyledons frozn each other. This layer is a portion of the decidua 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 labour be comes split asunder, so that while a portion is carried off along with the placenta, and constitutes its external membrane, the rest remains attached to tha inner surface of the uterus. This layer serves as a mediutn by which the uterine arteries (fig. 481. a a) and veins pass from the uterus into the placenta. Numerous valve-like apertures are observed upon all parts of the surface. They- are the orifices of the veins which have been torn off from the uterus. A probe passed into any of these, after taking an oblique direction, enters at once into the placental substance. Small arteries, about half an inch in length, are also everywhere observed embedded in this layer. After making several sharp spiral turns, they likewise suddenly open into the placenta. These are the uterine vessels, which convey the maternal blood to and from the interior of the placenta.