24. Another proof, amounting to demonstration, of the existence of a cortical covering to the human ovulum, and that of the filiform envelope called the shaggy chorion within it, I find in a striking and beautiful preparation belonging to Sir Charles Clarke, marked 87. We there see a complete cast of the lining (caducous membrane) of the uterine cavity after fecundation, thrown off at a very early period, and with it the ovulum, (of the size of a Portugal grape,) exhibiting the cortex ovi, shelly and membranaceous, of a dense texture, yet flexible, which has burst like the pericardium of some seeds, and allows the shaggy chorion beneath it to be seen. It is a curious fact, that even Ruysch has, in his fanciful manner, represented a human ovulum of about three weeks with the cortex burst, yet still in situ, and the shaggy chorion beneath it, in every way resembling the specimen of Sir Charles Clarke’s in the Museum of St. George’s Hospital.
25. Even the errors of some of the very able anatomists of the present day come in beautifully to corroborate Professor Boer’s observations of the cortical membrane. Breschet and Velpeau, for instance, state, that within what they (with their predecessors) have looked upon as the decidua in the human ovum, there is a cavity containing a peculiar liquid which, in their opinion, is essential to the nutrition of the fœtus. Now what is this but the very description of the ovulum by Boer (17), proving that there is a cortical membrane in the human ovum?
26. The cortical membrane is destined to be absorbed during the first months of utero-gestation, thus exposing the next membrane to the contact of the uterine lining (decidua), with which a connection takes place in that part where the placenta is to be formed. In that part, however, the cortex ovi is never altogether obliterated, but only made thinner; and, in process of time, it is converted into a mere pellicular envelope, which not only serves to divide the filiform vessels of the chorion into groups or cotyledons in order to form the placenta, but also covers all over those cotyledons or groups of vessels. (Plate I. Expl. of Fig. 1.) I have called this the membrana propria[[2]].
27. While the process or metamorphosis of the ovulum noticed by Boer takes place in the ovarium, in consequence of fecundation, the cavity of the womb does not remain idle, but forthwith sets about weaving for itself a general lining—a sort of pseudo-textile membrane—which extends all over the cavity, descends partly into the cervix, and is often, (not necessarily always,) projected even into a great portion of the fallopian tubes.
28. This adventitious lining of the cavity of the womb is formed quite independently of the presence of the ovum, for it has been found in most cases of devious gestation, where the fœtus was extra muros uteri (Plate VIII. IX. X. Fig. 1 and 2, and page [35]), and has been found advanced in its progress of formation, while the ovulum was, as yet, on its way through the fallopian tube after fecundation. (Haller, Lobstein, Velpeau, Meckel, Pockels.)
29. To this adventitious lining described by many ancient authors, but by none better than by Harvey, the name of decidua already mentioned (24, 26) has been given by Hunter, because, viewed as a production of the uterus it is caducous and not permanent like its own natural lining; as, however, it is no more caducous than any other of the involucra of the ovum, the denomination is not strictly correct. It is, nevertheless, more generally adopted than that of epi-chorion, given to the same pseudo-membrane by Chaussier.
30. Its mode of formation and its structure have been differently explained by different authors. The explanation offered by Burns runs counter to facts and realities. That author speaks of all the minute steps of generation, with the same confidence as if all had been demonstrated. His imaginings respecting primary and secondary vessels shooting out from the inner surface of the womb to form the decidua must be gratuitous, as no other anatomist or physiologist, either before or after him, has been able to ascertain the reality of such a process.
31. It is probable that the decidua consists of two laminæ, inasmuch as we always find it with one surface perfectly smooth and the other rough. If so, they are most intimately connected. It is at least one-twentieth of an inch in thickness during the first five or six weeks of uterofœtation, when its tissue is found to be more knotty, coarse, and full of short threads, (not unlike a very ordinary mat,) than a purely membranaceous or cloth-like lining would be. It is not until a more advanced period of gestation that the decidua becomes distinctly membranaceous, in which state it lines the entire cavity of the uterus. (Plate IX. and prep. 73, 75, 76, of Sir Charles Clarke’s Collection: also Dr. Agar’s beautiful specimen of impregnated uterus, Mus. Coll. Reg. Lond. Med., and several specimens in the College of Surgeons, particularly Sir W. Blizard’s case.)[[3]]
32. Into this chamber, then, so lined and so prepared the fecundated ovulum, in the condition described by Professor Boer, is received after its journey from the ovarium through the fallopian tube.
33. The pre-existence of an ovulum in the Vesiculæ Graafianæ, or Ovarian Vesicles—and its metamorphosis, after fecundation (14, 15, 16, 17, 18,) have been amply confirmed by Plagge of Bentheim, who published an account of his experiments and observations, illustrated by figures, in Meckel’s Journal of Physiology. (See also Prevost and Dumas, 3rd memoir.)