Corpus luteum. Upon impregnation taking place, one or more of the most prominent Graafian vesicles begins to show marks of considerable vascularity, both in its external capsule and in the surrounding stroma of the ovary. The vesicle swells, and at length bursts, discharging its contents into the funnel-shaped extremity of the Fallopian tube, which firmly grasps the ovary at this point by means of its fimbriæ.
These changes begin to take place immediately after impregnation; the inner lining of the vesicle, which Professor von Baer considers to be a mucous membrane, appears to undergo a rapid development, much more so than the external capsule which contains it. It is, therefore, thrown into a number of corrugations by which the cavity of the vesicle is greatly diminished; it becomes much thicker, and assumes a yellow colour. As its growth proceeds, the cavity of the vesicle becomes still farther contracted, until being unable longer to retain its contents, it bursts and discharges them as above described.
Corrugation of the lining membrane of the
Graafian capsule after impregnation.
From Baer.
The remains of the ruptured vesicle form a round glandular yellow coloured body, called corpus luteum: it projects considerably from the surface of the ovary, attaining the size of a small mulberry. In the middle of this projection there is a little irregular and generally triangular depression or indentation, which is the opening through which the ovum was discharged from the Graafian vesicle: this after a short time closes, forming a little cicatrix on the surface of the ovary.
Corpus luteum in the third month.
From Dr. Montgomery.
“Upon slitting the ovarium at this part, the corpus luteum appears a round body, of a very distinct nature from the rest of the ovarium. Sometimes it is oblong or oval, but more generally round. Its centre is white, with some degree of transparency; the rest of its substance has a yellowish cast, is very vascular, tender and friable, like glandular flesh. Its larger vessels cling round its circumference, and these send their smaller branches inwards through its substance: a few of these larger vessels are situated at the cicatrix or indentation on the outer surface of the ovarium, and are there so little covered as to give that part the appearance of being bloody when seen at a little distance.”[4] Upon making a section of a corpus luteum, we observe that its cavity has an angular form, from which, as from a centre, white lines radiate to the circumference of the vesicle; an appearance which is evidently produced by the corrugation of the inner membrane of the vesicle, as above alluded to. To a similar cause we may also attribute the lobular appearance, which the structure of the corpus luteum presents when a section is made of it. The number of these corpora lutea corresponds exactly with the number of newly formed ova. Meckel, after having examined no less than two hundred pregnant animals of the class mammalia, found that the number of corpora lutea corresponded exactly with that of the young produced. “When there is only one child,” says Dr. W. Hunter, “there is only one corpus luteum, and two in the case of twins. I have had opportunities of examining the ovaria with care in several cases of twins, and always found two corpora lutea. In some of these cases there were two distinct corpora lutea in one ovarium, in others there was a distinct corpus luteum in each ovarium.”
A Graafian vesicle cannot be converted into a corpus luteum except by actual and effective sexual intercourse; and the strange and discrepant accounts which have every now and then been published, even by authors of considerable repute, of corpora lutea having been found in the ovaries of virgin and even newly-born animals merely prove that the true characteristics of the corpus luteum were not sufficiently known. The irregular cysts, cavities, or deposites of whitish or yellowish structure which are frequently found in the ovary, independent of impregnation, and which have been improperly enough called virgin corpora lutea, present points of difference so marked that they can scarcely be mistaken by an experienced eye. The angular cavity opening externally, the stellated, radiated, cicatrix-like appearance, which a section of the corpus luteum presents, its soft and delicate structure as described by Dr. Hunter, and above all its vascularity, and the facility with which its vessels can be injected from the general tissue of the ovary, are characters only found in a true corpus luteum. Virgin corpora lutea frequently occur under circumstances of disease, especially those of a tubercular character. They frequently appear as distinct cysts, the walls of which are semi-cartilagenous; at other times they seem to be nothing more than a coagulum of blood: they seldom project much from the ovary, and in no instance have they the peculiar structure of the corpus luteum, nor the external cicatrix, nor are they capable of being injected.
After awhile the cavity of the corpus luteum contracts, and the opening into it closes. The surrounding stroma loses its vascularity, the prominence at this part of the ovary gradually subsides, and the ovary returns to its former size. The periods at which these changes take place vary, but with the exception of those first mentioned they proceed slowly whilst pregnancy lasts, after which time, now that the increased activity of the pelvic circulation peculiar to that period has ceased, they advance more rapidly.