Of the Formation of the Ovum.
The human uterus, in the unimpregnated state, consists of a succulent substance, in which we may perceive fibres running in every direction. In the interstices of these we find a serous fluid, which can easily be squeezed out. By injecting the vessels finely with size or mercury, we observe them to be numerously intermixed with the fibres, but very small. A portion of these vessels follow an irregular course toward the inner surface or cavity of the uterus, and open there upon the membrane which lines it. At the menstrual period the vessels enlarge or dilate a little, and their orifices become more distinct over the surface of the cavity, as may be ascertained by inspecting those who have died at this time. Sometimes a slight temporary serous secretion precedes the flow of the menses, and succeeds it. More frequently the vessels yield a bloody fluid at once, which continues a certain number of days.—When impregnation takes place, the vessels enlarge still more; and we can observe small trunks forming in the substance of the uterus, the largest of which are at the two sides where the spermatic and hypogastric branches meet[[1]]. The old fibres are more separated, and new ones added, especially at the fundus.—The quantity of interstitial fluid is also increased, so that the uterus becomes manifestly softer. The vessels, as they enlarge, tend to the uterine cavity; but instead of opening there, and yielding a fluid, as in menstruation, they either elongate themselves, or, which is more probable, form new and very delicate vessels, which project from the inner surface of the uterus, giving it an appearance as if covered with down. This takes place first and chiefly at the fundus, and, in a slighter degree, immediately above the narrow cervix, whilst the intermediate body remains still quite smooth.
These vessels project for about a line in length, at right angles, from the surface which yields them; and as they consist of arteries and veins, the down, or efflorescence, which they form, has, after death, a striated or radiated appearance, the empty arteries being white, the fuller veins black or red. These vessels constitute the outer layer of the decidua, or what may be called decidua striata, although the stria be obliterated as gestation advances.
Almost immediately after the formation of these primary vessels, they secrete from their extremities a membrane, or rather an irregular tissue of vessels, which, on account of their origin, may be called secondary. These assume a direction at right angles to those which formed them, so that they cross the stria or primary vessels, and therefore any body coming down through the first set of vessels, must, before it can get into the cavity of the uterus, either rupture this secondary organ, or push it before it. It is in this manner that the inner layer of decidua is formed, part of which is afterwards protruded before the vesicular ovum, constituting decidua reflexa, or protrusa.
The primary vessels adhere pretty closely together at their roots, but are more loose or separated at their termination. They are at first only yielded by the fundus, and in a small degree by the surface, immediately above the cervix, whilst the cervix itself produces from the lacunæ, which are increased in magnitude, a jelly, which sometimes fills up the os uteri completely; at other times there is only a small quantity formed in the cervix, leaving the os uteri quite patent. The inferior part of the uterine surface, which yields the primary vessels, is not a quarter of an inch in breadth; and the cavity being there small and narrow, the vessels from the opposite sides soon come in contact, and intermix without forming any secondary vessels. This portion may be called the cervical efflorescence.
The secondary vessels are very different from the primary; for, whilst the latter are short, straight, and parallel to each other, the former are more extended, intermix, and ramify together, so as to form an irregular tissue or sheet of vascular substance, the fibres or vessels of which assume a direction at right angles to the down or primary vessels which formed them. This direction seems to be very naturally produced by the weight of the secondary vessels, which makes them hang down or point to the os uteri. Very soon we can perceive ragged irregular processes, hanging into the cavity of the uterus, and extending toward the cervix, so that this layer of decidua presently appears to consist of a number of torn floating membranes, like portions of spiders’ webs, pendulous in the uterine cavity; but nearer examination shows that there has been no laceration, the margins being smooth and well defined. These I would call the processes of the decidua interna.
Such is the structure of the decidua externa and interna, at three weeks or a month after impregnation; and at this period no fœtus is in the uterus. In one case I detected the vesicular ovum in the tube. It was about half way betwixt the two extremities, was rather smaller than a full-grown pea, and contained a little fluid. In another case I found it still in the ovarium, covered by the fimbriated extremity of the tube. By puncturing the peritoneal coat of the gland, the vesicle escaped.
It has been the general opinion, since the time of Dr. Hunter, that the decidua extended a little way into the tube; and on his authority I stated this to be the case in an account which I formerly published. By careful examination, I am now convinced that this is not the case. My brother, in his dissections, even thought that the uterine extremity of the tube was less changed in point of vascularity, than any other part of it.
When the fœtus does descend into the uterus, it is contained in a double membrane. The internal one is the amnion, and possesses no distinct vessels. The external one is the chorion, and is from the first vascular, and soon becomes so much so, that its vessels have by some been described as forming a distinct tunic.
From what has been said, it will be evident, that, when this vesicle does reach the uterus, it will be received amongst the primary vessels which will surround it, whilst the secondary vessels, or decidua interna, will lie before it, and prevent any communication betwixt it and the uterine cavity. But it cannot remain long thus; for, in proportion as the vesicular part of the ovum increases, it will push the decidua interna before it, and encroach upon the cavity of the uterus. This circumstance, together with the intimate connection formed betwixt the vessels of the chorion and those of the decidua, with the consequent production of a placenta, I have already very fully detailed in a former publication, to which I refer[[2]].