By the great number of dissections which I performed in the course of this month, I was every day confirmed in my opinion that the carunculæ of the uterus perform the office of the placenta; they are at this time found of a reddish colour, turgid, and of the size of walnuts. The conception, which had previously adhered to the caruncles by the medium of mucor or glutinous matter only, now sends the branches of its umbilical vessels into them, as plants send their roots into the ground, by which it is fastened and may be said to grow to the uterus.
About the end of December the fœtus is a span long, and I have seen it moving lustily and kicking; opening and shutting its mouth; the heart, inclosed in the pericardium, when exposed, was found pulsating strongly and visibly; its ventricles, however, were still uniform, of equal amplitude of cavity and thickness of parietes; and each ending in a separate apex, they form together a double-pointed cone. Occasionally I have seen the fluid contained in the auricles of the heart, which at this time present themselves as ample sacs filled with blood, continuing to pulsate for some short time after the ventricles themselves had left off contracting.
The internal organs, all of which had lately become perfect, were now larger and more conspicuous. The skull was partly cartilaginous, partly osseous. The hooves were yellowish, flexible, and soft, resembling those of the adult animal softened in hot water. The uterine caruncles, of great magnitude and like immense fungi, extended over the whole cavity of the uterus, and plainly performed the office of placentæ, for numerous and ample branches of the umbilical vessels penetrated their substance there to imbibe nutritive matter for the growth of the embryo. As in the fœtus after birth, the chyle is now carried by the mesenteric veins to the porta of the liver.
Where there is a single fœtus the umbilical vessels are distributed to the whole of the carunculse, both those of the horn where the fœtus is lodged and those of the opposite horn; where there is a pair of embryos formed, the umbilical vessels of each only extend to the caruncles of the horn appropriated to it.
The smaller umbilical veins in tending towards the fœtus, form larger and larger trunks by coalescing, until at length two great canals are formed, which in conjunction pour their blood into the vena cava and vena portæ. But the umbilical arteries, which arise from the division of the descending aorta, form two trunks of small size, not remarkable save for their pulse: proceeding to the boundary of the conception, in other words, to the conjunction of the placenta or carunculæ with the ramifications of the umbilical veins, they first divide into numerous capillary twigs, and then are lost in others that are invisible.
As the extremities of the umbilical veins within the uterus terminate in the caruncles, so the uterine vessels on the outside, which are large and numerous, and bring the blood from the mother towards the uterus, by means of the vessels of the suspensory ligaments, terminate externally on the caruncles. It is to be noted, also, that the internal vessels are almost all veins; the external vessels, again, are in many instances branches of arteries. In the placenta of the woman, if it be carefully examined immediately after delivery, a much larger number of arteries than of veins, and these of larger size, will be found dispersed on every side in innumerable subdivisions to the very edge of the mass. In the same kind of spongy parenchyma of the spleen, the number of the arteries is also greater than that of the veins.
The exterior uterine vessels run to the uterus, as I have said, not to the ovaries (testiculi) situated in the suspensory ligament, as some suppose.
I have remarked an admirable instance of the skill of nature, in the bulge or convexity of the caruncles turned towards the conception: a quantity of white and mucilaginous matter is discovered in a number of cavities, cotyledons, or little cups; these are all as full of this matter as we ever see waxen cells full of honey; now this matter, in colour, consistency, and taste, is extremely like white of egg. On tearing the conception away from the caruncles, you will perceive numbers of suckers or capillary branches of the umbilical veins, looking like lengthened filaments, extracted at the same time from every one of the cotyledons and pits, and from amidst their mucilaginous contents; very much as we see the delicate filaments of the roots of herbs following the stem when it is pulled out of the ground.
It is clearly ascertained from this that the extremities of the umbilical vessels are not conjoined by any anastomosis with the extremities of the uterine vessels; that they do not imbibe any blood from them, but that they end and are obliterated in that mucilaginous matter, and from it take up their nourishment, nearly in the same way as at an earlier period they had sought for aliment from the albuminous humour contained within the membranes of the conception. In the same manner, consequently, as the chick in ovo is nourished by the white of the egg through its umbilical vessels, is the fœtus of the hind and doe nourished by a similar albuminous matter laid up in these cells, and not directly from the blood of the mother.
These carunculæ might therefore with propriety be called the uterine liver, or the uterine mammæ, seeing that they are organs adapted for the preparation and concoction of that albuminous aliment, and fitting it for absorption by the veins. In those viviparous animals consequently that have neither caruncles nor placentæ, as the horse and the hog, the fœtus is nourished up to the moment of its birth by fluids contained within the conception or ovum; nor has the ovum in these animals at any time a connexion with the uterus.