Functions of the placenta.— By means of the placenta, the blood of the mother is brought into mediate relation with that of the fcetus. Two currents, the one fcetal, and the other maternal, are continually flowing into and from this organ, yet in channels so perfectly distinct that no direct commingling of the streams can ever take place. Never theless, though no passage of the form-ele ments of the blood can occur, yet through the partition-walls which separate these ow currents, all the materials necessary to the growth of the fcetus are conveyed by endos motic processes, and all the changes necessary to the respiration of the fcetus, and to the elimination of effete materials, are effected. The mechanism by which these ends are accomplished is of two kinds. The one consisting of means for bringing the two con stantly flowing streams of blood into juxta position, the other of instruments for carrying on the nutritive and eliminative processes which are the objects of this conjunction.
The fcetal blood conveyed by the branches of the two umbilical arteries, is distributed to the villi, whence, after being exposed, in the finer vessels and capillaries which ramify upon their inner surface, to the influence of the maternal blood, it is returned to the funis by the branches which terminate in the single vein. The propelling power by which the blood is moved resides in the heart of the fcetus, and the whole of its circulating fluid is thus carried in successive portions through the placenta.
The maternal blood, after having its im petus diminished by the spiral course which the arteries take in passing through the walls of the uterus, as well as through the decidua, is delivered at once into the placenta, where it becomes immediately separated into fine streamlets by the villi which are so closely set a.s to break up the interior of the organ into countless channels. After flowing every where among the villi, the blood escapes back into the uterine system* by the venous pri7 fices upon the surface of the decidua, and upon the dissepiments and marginal furrows from which it is conducted, through the deci dual coat, to the sinuses in the substance of the uterus, and thence is returned to the mother's body by the uterine and spermatic veins.
During the flow of these streams through the interior of the placenta, the surface of the villi is constantly bathed by the maternal blood. Nevertheless the blood of the fcetus is separated from that of the mother — first, by the walls of its own capillaries ; secondly, by the gelatinous membrane in which these ramify ; and thirdly, by the external non-vas cular nucleated sheath derived from the cho rion. With the latter alone, the maternal blood is brought into direct contact.
Each of these structures has its distinct office. The use of the external layer of cells (fig. 485. b) has been happily illustrated by Goodsir. They are to the ovum what the spongioles are to the plant : they supply it with noutishment from the soil in which it is planted. Thus their action is selective, and they transmit to the interior of the villus the 'materials necessary for fcetal growth. These again are taken up by the internal layer of cells (fig. 485. b), and by them brought into direct contact with the fcetal capillaries. By a similar process, the interchanges necessary- to respira tion are effected through the membranous sur faces which separate the maternal and fcetal blood. And these processes, respiratory and nutritive, are continued without intermission from the moment that the two separate cur rents are established until the final separation of the fcetus in the act of birth. Yet, through
out pregnancy, the form of the mechanism by which these changes are effected is continually altering, either in its greater or lesser parts. The greater changes have reference chiefly to mechanical, and the lesser to vital necessities. The changes in form exhibit a beautiful series of,adaptations in ,the.capacity and strength of the placenta to the increasing amount and force of the maternal current. The original plan of the placenta, that of an interspace be tween two spheres (a lesser one contained within a greater) filled by maternal blood, eould not be long preserved vvith the materials out of which the temporary organ is eon Structed. For as the ovum grows, the deci dua reflexa, which alone confines the blood that flows around it, becomes thinner, _and finally gives way by extension. But long be fore this stage arnves, the whole of this por tion is shut out from the maternal circulation, and the subsequent metamorphoses are di rected to the strengthening of the more limited space which remains. It is on this account that the strong border of decidua is formed around the margin of the now restricted area. The base of the placenta now consists of the tough and resisting chorion; while that por tion alone of the decidua which is strength ened externally by the uterine walls is retained to form the opposite boundary. Ultimately, as the current of maternal blood flows with increasing force into the placenta in propor tion to the growth of the latter, this becomes subdivided by the decidual septa, which ap portion the entire organ into separate placen tulx, and thus the larger supplies necessary to the increasing exigencies of the fcetus are disposed of without danger of rupture to any portion of the organ.
The changes in the more minute structures which belong to the fcetus are not less inter esting. The profuse development of fine capillaries within the foetal tufts, which is so conspicuous from the third to the sixth month, is connected not only with the functions of respiration and nutrition of the fcetus, but also with the growth of the villi themselves. But when the period of viability of the fcetus has arrived, the proportionate amount of capillary vessels within the villi becomes greatly re duced, until finally only the original stems of the vessels are left. And this relative reduc tion of the channels through which the fcetal blood flows, becomes naore marked, until, as the time of birth approaches, many of the villi become more or less obliterated, and cease to admit blood, often in consequence of that calca reous degeneration which, from the frequency of its occurrence, may be regarded rather as a normal process significant of natural decay than as an evidence of any morbid or preter natural change.
The series of metamorphoses is closed by the degeneration of the materials which bind the placenta, and consequently the fcetus, to the uterus. The layer of decidua forming the connecting medium between the uterus and the fcetal structures, in common with the rest of this membrane, suffers slow disintegration, and its component cells are converted into molecular fat. And now the strength of the adhesion being gradually diminished, it only remains for the contractile power of the ute rus to be evoked in order to accornplish the separation together of the fcetus and placenta, like ripe fruit detached from the parent bough.