2letamorphosis and restoration of the com ponent tissues. — The first and immediate re duction in size of the uterus, after parturition, has been just stated to depend upon mere contraction of the uterine fibre. But con traction alone will not account for those great and remarkable reductions in the dimensions of the organ which have been just described. The true explanation of these phenomena is furnished by a series of metamorphoses affect ing more or less the entire uterine tissues, by which the greater portion of those structures which have been formed during pregnancy, become disintegrated and removed, while other and new tissues are developed in their place.* In these metamorphoses, the colossal fibre cells, which form the great bulk of the newly added material, play the most important part. These have been traced, during their develop ment in pregnancy, from the small fusiform cell of the unimpregnated uterus to the fully formed fibre of the organ at term. The growth of these proceeds pari passu with that of the fcetus, for whose expulsion they are destined ; and this act being accomplished, their de struction and removal becomes a necessary prelude to the recomposition of the entire organ upon the same type as before 'impreg nation. In this respect, the aggregate forma tion of fibre cell is comparable to the deer's horn, the placenta and other structures which, having served the purpose of their formation, and being incapable of suffering retrogression, become caducous, with this difference, how ever, that the one class of structure being thrown off in a mass, the act of separation is striking and obvious ; while the deciduous process in the other is gradual and fragmental, and can only be discovered by the most pa tient and careful scrutiny.
The disintegration and removal of the ute rine muscular fibre is effected, first, by the transformation of each fibre into molecular fat. This process does not commence earlier than the fourth or sixth day after labour, and not later than the eighth day. Certain dif ferences are observable in the order of retro gression. Thus the process begins somewhat later in the inner than in the outer laminm, while the cervix remains unchanged a few days longer than the body. In the individual fibres, the process of decay begins at many points simultaneously. The fibres lose their sinuous outline, and become paler ; while within them appear yellow oil granules, commonly arranged in rows. The nucleus of the fibre is pale, but distinct, until it becomes obscured by the increase of the oil granules ; while the extremities of the cells, on account of their tenuity, are the first to suffer disin tegration.
During the second and third week, the process of disintegration continues ; mid it is probable that a considerable absorption of effete material now takes place, since it is not easy to explain otherwise that rapid diminu tion in bulk, especially in the second week, which the entire organ undergoes, as shown by the calculation of weights already given. As a result of these molecular changes, the uterus now loses its reddish colour, and be comes of a dirty yellow, and is at the same time more easily lacerable.
In the course of thei fourth week, and pos sibly sometimes durag' the third, there ap pears, in the midst of the now degenerated fibres, the first traces of a new formation of uterine substance. These occur first in the
form of cell nuclei, which are concurrently developed at several points ; and gradually, while the last portions of the old muscular coat are being disintegrated and absorbed, acquire the character of the new muscular fibre cells (fig. 455. b). So that, by the end of the second month, the reconstruction of this portion of the uterine substance is often com plete.
The disintegration of the remains of the decidua, and the reconstruction of the lining membrane of the uterus, which had been re moved during the act of birth, is effected by a process very similar to that just described.
With regard, first, to that portion of the inner uterine superficies, which had been co vered by the placenta, it is observed that this undergoes a somewhat slow retrogression. The veins, filled by thick clots in the normal state in consequence of the progressive invo lution of the intermediate uterine substance, occasion here a marked protrusion ; so that very often, after four or six weeks, the pla cental snace forms an elevated snot of twice the circumference of a dollar. Finally, how ever, these coagula are removed, and, together with the veins, disappear, while the place sinks to the level of the surrounding parts ; and, after becoming smooth and receiving an in vestment of mucous membrane, is generally no longer discernible. The restoration of the placental space to its former condition does not, however, always proceed nornially. Sometimes, in consequence of excessive acti vity in the process of reconstruction, hyper trophic growths of the new material take place; so that, several months after labour, a tumour of more or less considerable size, formed at the expense of the uterine tissues, is found to occupy the original seat of the placenta. I have satisfied myself by several microscopic examinations of the correctness of Heschl's opinion, that in this way are formed some of those anomalous-looking fleshy substances which are occasionally dis charged from the uterus, and are regarded as moles.
The histological changes, which take place after labour in the tissues lying internally to the muscular coat, up to the complete resto ration of the mucous membrane, have been examined by many observers, not always, however, with corresponding results. It ap pears certain that a portion at least of that layer of decidua which is still left attached to the uterine walls, is removed by fatty trans formation, and that many of the products are discharged by the lochia. Schrceder van der Kolk has traced this process as it occurs in the nuclear cells and fibres, which form so large a portion of the decidua. Those very broad fibre cells, which are visible in it up to the ninth month of pregnancy, are no longer to be found four or five days after labour, when they appear to be transformed into long cells, through an abundant fatty transforma tion which progressively continues, until, by the increasing development of the oil granules and the corresponding diminution of the cells and fibres, the situation of the latter can ulti mately only be discovered by the still existing longitudinal direction of the fat nuclei, while all traces of a cell wall have entirely disap peared.