The placenta in the woman grows to the upper part of this membrane. In the sheep, numerous carunculæ adhere to it at various points. In the fallow and red deer the ovum is united to the uterus in five places only; whilst in the mare it is in contact with the inner surface of the uterus by an almost infinite number of points of attachment. Hence Fabricius[384] states that in almost all viviparous animals there is a soft, loose, porous, and thick fleshy body of a dark colour, in intimate union with the terminations of the umbilical vessels; he compares it to a sponge, or to the loose parenchyma of the liver or spleen; hence, too, it was called by Galen[385] “glandular flesh;” and it is now commonly known by the name of the uterine liver, in which the extremities of the umbilical vessels ramify to bring nutriment from the uterus to the fœtus.
But this fleshy substance is not found in all animals, nor at all periods of utero-gestation; but in those alone in which the conception adheres to the uterus; and then only when it becomes attached for the purpose of taking up nutriment. At the commencement the “conception” (like an egg placed within the uterus) is found in contact with every part of the uterus, yet at no point is it adherent; but produces and nourishes the embryo out of the humours contained within it, as I have explained in the instance of the hen’s egg. This adhesion, or growing together, first takes place, and the fleshy mass (constituting the bond of union between the “conception” and the uterus) is first produced, when the fœtus becomes perfectly formed, and, through want either of different or more abundant nourishment, dispatches the extremities of the umbilical vessels to the uterus, that from hence, (as plants do from the earth by their radicles) it may absorb the nutrient juices. For in the beginning, as I have said, when the “punctum saliens” and the blood can alone be seen, the ramifications of the umbilical vessels are only visible in the colliquament and amnion. When, however, the fabric of the body is completely formed, the ramifications extend further, and are distributed in vast numbers throughout the chorion, that from the albuminous fluid which there exists, they may obtain nourishment for the fœtus.
Hence it is manifest that the young of viviparous animals are at the beginning nourished in exactly the same manner as the chick in the egg; and that they are detained within the uterus in order that (when they can no longer supply themselves with nutriment from their own stores) they may form adhesions to it by means of this fleshy substance, and receiving more abundant supplies of food from the mother, may be nourished and made to grow.
Wherefore Fabricius has rightly observed, that in some animals the “conception” is scarcely attached to the uterus at all. Thus the sow and the mare have no such fleshy mode of union,—but in them the ovum or “conception,” as in the beginning it is formed out of the humours of the uterus, so it is nourished subsequently by the same means; just as the ovum of the hen is supplied with aliment at the expense of the albuminous matter without any connexion whatever with the uterus: and thus the fœtus is furnished with aliment by the “conception” in which it is contained, and is nourished as the chick is from the fluids of the egg. This is a strong argument that the fœtus of viviparous animals is no more nourished by the blood of the mother than the chick in the egg; and moreover, that the fluid within the chorion is neither urine nor any other excrementitious matter; but serves for the support of the fœtus. Although, as I have before remarked, it is possible when all the nutrient portions have been taken up, the remainder may degenerate into excrementitious matter resembling urine. This is also clear from what I formerly observed of the cotyledons in the deer, viz., that in these animals the fleshy mass was of a spongy character, and constituted, like a honeycomb, of innumerable shallow pits filled with a muco-albuminous fluid, (a circumstance already observed by Galen[386]); and that from this source the ramifications of the umbilical vessels absorbed the nutriment and carried it to the fœtus: just as, in animals after their birth, the extremities of the mesenteric vessels are spread over the coats of the intestines and thence take up chyle.
Of the Placenta.
In my opinion, then, the placenta and carunculæ have an office analogous to that of the liver and mamma. The liver elaborates for the nourishment of the body, the chyle previously taken up from the intestines: the placenta, in like manner, prepares for the fœtus alimentary matters which have come from the mother. The mammæ also, which are of a glandular structure, swell with milk, and although in some animals they are not even visible at other times, they become full and tumid at the period of pregnancy; so, too, the placenta, a loose and fungus-like body, abounds in an albuminous fluid, and is only to be found at the period of pregnancy. The liver, I say, then, is the nutrient organ of the body in which it is found; the mamma is the same of the infant, and the placenta of the embryo. And just as the mother forms more milk from her food than is requisite to sustain her own flesh and blood, which milk is digested and elaborated in the mamma; so do those animals, furnished with a placenta, supply to the fœtus nourishment which is purified in that organ. Hence it happens that the embryo is furnished with good or bad nutriment just as the mother takes wholesome or unwholesome food, and in proportion as it is elaborately prepared or not in these uterine structures. For some embryos have a more perfect structure provided for them, such as that fleshy substance mentioned above, which in some is altogether wanting. In some, also, the placenta is observed to be thicker, larger, and more loaded with blood; whilst in others it is more spongy and white, like the thymus or pancreas. But there is not more variety found in the placenta than in the mamma or viscera generally: for instance, the liver in some animals is red and filled with blood, in others, as is the case with fishes and some cachectic persons in the human species, it is of much paler hue. The mare feeds on crude grass, and does not ruminate; the sow gorges itself with any unclean food; and in both the placenta, or organ for perfecting the aliment, is wanting.
Rightly then is it observed by Fabricius,[387] that “this fleshy structure, differs much in shape, size, position, and number in different kinds of animals. The human female has one placenta only; as is the case with the mouse, rabbit, guinea-pig, dog, and cat:” so also with many animals which have the toes distinct, and incisor teeth in both jaws. “All those which have the hoof cloven and incisors in one jaw only, have several placentæ, whether they be domesticated animals, like the sheep, cow, and goat, or wild, as the red-deer, roe, fallow-deer, and others of the same kind. Again, where there is only one of these fleshy structures it either resembles a cake, (whence its name placenta), as in the human female, rabbit, hare, mole, mouse, and guinea-pig; or else it is like a girdle or bandage encircling the trunk of the body, as in the dog, cat, ferret, and the like.” In some it resembles the cup or chalice of the acorn, and surrounds the greater part of the “conception,” as in the hare and rabbit, its convex part adhering to the uterus, the concave looking towards the fœtus. “Again, in animals which have this structure in the form of a cake, although the shape is similar, the situation in which it is found is very different. In the human female it adheres to the fundus of the uterus, and is as far removed from the fœtus as possible, their connexion being effected by means of long vessels. In the mouse, guinea-pig, and rabbit, it is attached partly in the region of the loins, partly at the sides of the thorax. Those animals which have numerous placentæ are all furnished with incisors in one jaw only, as the sheep, cow, goat, red-deer, roe, and the like. Yet in these some variety is observable.”
For in the sheep the carunculæ are many in number, and of different magnitudes, the largest being of the size of a nutmeg, the smallest of that of a pea or vetch: they are also of a rounded form and reddish hue, with their convex portion turned towards the uterus, something in the semblance of soft warts or nipples. “In the cow they are larger, wider, and whiter, more like a spongy or fungoid body,” and they appear to take their origin from the chorion. In the red or fallow-deer they are five only in number; these spring from the walls of the uterus, and thrust themselves inwards, exhibiting their depressions or acetabula on the side of the fœtus. But in all animals it is observed that the carunculæ adhere firmly to the uterus, and cannot be separated from it without considerable difficulty, except at the period of birth; at which time they become loosened from their attachments and fall like ripe fruit. If they are forcibly torn from the uterus, I have observed the greater part of the blood that escapes to flow, not from the “conception,” but from the uterus itself.
Fabricius,[388] when discussing the mode of union between the “fleshy substance” and the uterus, uses many arguments, but in my opinion weak ones, to prove that the umbilical vessels anastomose with those of the uterus: yet he seems chiefly to have done so to countenance the old opinion once held almost by all; for he confesses that he can make no positive assertion on the subject, “because the fleshy mass itself stands in the way of any accurate investigation.” Yet neither reason nor observation would lead us to believe that more anastomoses exist in the uterus, than in the liver between the branches of the vena portæ and the cava; or in the mamma, between the vessels which transmit blood and those which carry milk. There may be, indeed, at places a juxtaposition of vessels, and sometimes the insertion of one into the coats of another; but the perfect coalition and union, described by Fabricius, never exist. Were it so, the veins and arteries ought to be continuous; for the vessels which bring the blood from the mother into the uterus and carunculæ are arteries, whilst those which pass from the uterus to the fœtus are veins, as is readily apparent; for they carry blood from the placenta into the vena cava.
Hence the opinion of Arantius seems to me to be the true one, viz. that the orifices of the umbilical veins are in no way continuous with the uterine vessels. For there is a smaller number of vessels carrying blood to the uterus than there is of veins returning it to the fœtus; and the greater part of the roots of those terminate in the chorion. Yet Fabricius, either from respect to the ancients, or through an envious feeling towards Arantius, most pertinaciously holds to the old opinion.