Another question which has never been satisfactorily determined, relates to the ulti mate fate of the decidua reflexa. Dr. Hunter, from observing that, at the time of birth, only one layer of decidua can be found upon the secundines, supposed that, after a certain pe riod of pregnancy, the decidua vera and re flexa, having come into contact, united to form one membrane. Doubting this explana tion, I have made many observations, with a view to settle this point ; and from these I am satisfied that no such union takes place ; but that, when the decidua reflexa has ful filled the offices already assigned to it, and has ceased to be vascular, so that no further addition of material to it can take place, it becomes, after the fifth or sixth month, so completely attenuated by distension from the growth of the ovum within, that it is reduced to a mere film, of which the only trace left at, or indeed before, birth, is a narrow frill still discoverable at the margin of the placenta between the decidua vera and the chorion. But the decidua lining the uterine walls con tinues vascular to the last ; and this alone constitutes the membrane a part of which at birth is found adherent to the outer surface of the chorion, and which Dr. Hunter, from ob serving that it now consisted of only one layer, imagined was formed of the two de cidux united together.
.Histology of the decidua. — The morpholo gical changes effected during pregnancy in the decidua, and the chief purposes of these, hav ing been stated, the histological peculiarities will now be briefly described. The lining membrane of the uterus, from infancy on wards, is composed, as already shown, of free elementary corpuscles or nuclei, contractile fibre cells, amorphous tissue and epithelium, together with capillary vessels, and the tor tuous canals termed uterine glands. These undergo important modifications, which serve to explain the great and rapid growth of the decidua during pregnancy. Accordina to Schrceder van der Kolk, who has traced' and figured with great care the several stages of development of these elemental tissues, the cells of the decidua, surrounding an ovum of about three weeks, situated nearest the villi, have already undergone considerable enlarge ment. These occurred in the form of oval nucleated cells Cfig. 4.54. B a), vi ith fine nuclei and fat granules, b, intermixed ; while in the layer of the decidua, still deeper, oc curred longer cells, that were already begin ning to form fibres.
In an ovurn of five weeks, similar cells were found, in a further stage of development. In the superficial decidual layers, the oval cells, C a, were filled with granules, and contained a nucleus, and some a nucleolus. In the deeper layers, as before, the cells had become more elongated, C b. In and between all
these cells were numerous minute fat granules, and amona the cells lay fine nuclei. The openings of the utrieular glands, D, which were surrounded by enlarged epithelial cells, were now considerably expanded, as compared with their usual condition previous to im pregnation, A.
At two months, the increase in size of the oval cells, E a, now abundantly furnished with fat granules, was still more marked. These were developed into long cells, b, which were found composing those valve-like rnembra nous septa formed now everywhere on the deeper decidual layer, as already described,fig. 152.
From this period onwards, the developrnent of the cells proceeds more and more rapidly, until those in the deeper layers become trans formed into fibres, which it is impossible to distinguish, under the microscope, from the peculiar contractile fibre cells of the true mus cular structure.
In the ninth month are found colossal fibre cells, le a, which are rarely seen beyond the margin of the placenta. These were very transparent, and exhibited, sorne one, and some two, nucleoli. A remarkable three pointed cell is sometimes also observed, F b. Fibres of fibrous tissue occur everywhere, and between them small cells and nuclei. The utricular glands have long ceased to be dis cernible in the advanced stages of pregnancy-.
According to the observations, however, of Bolin, Robin, and Kilian, from the fourth or fifth month onwards, the decidua begins to lose the character of ener„aetic life, which, up to that period, it had exhibited, and becornes atrophied, and less firrnly adherent to the uterine walls ; hile, between it and the mus cular parietes, there appears a new formation of decidua, at first soft and delicate, but which gradually acquires the peculiar characteristics of that metnbrane. This layer is not thrown off at birth, nor dispersed in the lochia, but remains attached to the inner uterine surface, and forms the foundation of the new mucous membrane, with which, after labour, the ute rus is furnished. M. Robin supposes that this new soft layer is often mistaken for a product of inflammation occurring in puer peral and other uterine maladies.
e. The uterus after parturition, Immediately after labour, the uterus, if entirely empty, occupies the whole of the pelvic cavity, together with the lower portion of the hypogastric region. The bulk of the organ varies in different individuals, and is considerably greater after twin or multiple pregnancy.
The tissues generally are of a redder colour, and softer, and more easily lacerable than in the unimpregnated condition ; those of the cervix being usually more lax than those of the body, from infiltration of serum, and oc casionally, in parts, of blood.