Notwithstanding these arguments, it may possibly be imagined by some that the humours which I believe serve for the nutrition of the fœtus are excrementitious, led chiefly by the fact that they increase as the fœtus grows larger, and in some animals are observed to exist in immense quantities at the period of birth (at which time it might be supposed that all alimentary matters would have been absorbed), and serve besides other uses hardly compatible with their supposed function of nutrition. I nevertheless most confidently assert my belief that these humours are at the commencement destined for the nourishment of the fœtus, just as the colliquament and albumen are in the case of the chick; but that, in course of time, when the thinner and purer portions are absorbed, the remainder takes on the character of excrementitious matter, but still has its uses, and in some animals especially conduces to the safety of the fœtus, and also greatly facilitates birth. For just as wine becomes poor and tasteless when the spirit has evaporated; and as all excreted matters owe their origin for the most part to what has been previously food; so, after all the nutrient portions of the fluid contained in the chorion have been taken up by the fœtus, the remainder become excrementitious, and is applied to the above-mentioned uses. But all the fluid of the amnion is usually consumed by the time of birth; so that it is probable the fœtus seeks its exit on account of deficiency of nutriment.
Lastly, if any other fluid is ever contained within the allantois, and this is sometimes the case, I believe it to be unnatural. For sometimes we see women at their delivery have an enormous flow of water, sometimes a distinctly double flow; and this the midwives call the “by-waters.” And so some women are seen with the abdomen immensely distended, and yet they bring forth a little shrivelled fœtus accompanied by a vast flow of water. Some imagine that a larger quantity of water is found with weakly and female children, whilst stronger and male fœtuses have a smaller share. I have often seen the waters come away in the middle of pregnancy, and abortion not take place, the child remaining strong and vigorous until birth. Since, then, there are naturally two collections of fluid, one in the chorion, the other in the amnion, so it sometimes happens that unnatural accumulations take place either in membranes of their own, or between the duplicatures of the chorion.
Of the Membranes.
With respect to the membranes or tunics of the uterus; as their special office is to contain the “waters,” and as these waters are two only, it is pretty certain that the membranes themselves do not necessarily or usually exceed that number.
Those who enumerate three tunics are, I believe, in error, owing to the ancients having described the same membrane at one time under the title of “chorion,” from the concourse of veins, at another under the name of “allantois” from its form.
Unquestionably, every “conception” is inclosed in two envelopes, just as the brain is surrounded by its two membranes; every tree and fruit, moreover, has its double bark; and lastly, seeds and fruits are protected by a double covering, the outermost of which is harder and stronger than the inner one.
Of the above-mentioned membranes, the innermost (that which contains the colliquament or purer fluid,) is exceedingly delicate; it is called the “amnion,” i.e. the mantle, from the way in which it is disposed round the fœtus. The outer tunic, however, is much thicker and stronger, and has received the name of “chorion,” “because,” says Fabricius, “a multitude of arteries and veins are aggregated together and arranged in it, as it were, after the manner of a chorus. Hence one of the tunics of the eye has been denominated χοροειδὴς (choroid) from its vessels having a similar arrangement to those in the chorion; the plexus of arteries and veins in the ventricles of the brain has also gained its name from the same circumstance.”
The chorion fills the whole uterus, and contains a viscid and rather turbid fluid; whilst the placenta, or carunculæ, adhere to its outer surface, and thus attach the “conception” to the uterus.
In the woman it is usually adherent to the amnion at its lower portion; nor can it be separated there without difficulty. In cloven-footed animals the chorion is of very large size, and contains a hundred times more fluid than the amnion: this last membrane at first is scarcely as large as a nutmeg, or broad bean, and is generally found in one or other horn of the uterus; that, namely, in which the embryo lies.
In the woman, more particularly, the chorion is externally rough and viscous, but internally it is smooth, slippery, and interwoven with abundance of vessels. In the woman, also, the upper part is thick and soft, but the lower is thinner and more membranous in character.