In the meantime the lower vaginal third of the genital canal has be come dilated and folded upon itself. The margins of the canal, which project first into the cloac,a and then into the sinus urogenitalis, are trans formed into the hymeneal ring, thenceforward forming the sharp dividing line between the external and the internal genitals.
We have seen that the middle third of the genital canal becomes changed into the uterus before the union of the two lateral halves is com plete. Nevertheless, there is no trace of the division by the end of em bryonal life under normal circumstances. This complete union, how ' ever, is peculiar t,o man and the quadrumana; in all the lower animals a greater or less division remains. Both Mullerian ducts may remain en tirely separated, or, if united at their external orifices, may fornt two complete uteri and vaginte. This occurs with many rodents, as in the hares. In other animals not only the vagina, but the lower part of the uterus also, is single, while the greater portion is divided into two separate horns. This is the case with the mouse. In others, as in the earnivora, there is a common uterine cavity, while the upper portion of the organ is bi-lobed. In the chiropterm and prosomim the uterus is simple, but shows traces of its bi-lateral origin by two short caruna, thus forming the connecting link between the lower orders and the monkeys.
In other animals again one of the Mfillerian ducts remains undevel oped or disappears, as also does the corresponding ovary. This is the case with birds.
All these modifications are of interest to us, since what we see occur regularly in the lower animals may happen as an abnormality in the human being.
The simple uterus at birth has by no means the form of the sexual) y mature organ. The cervix forms the chief mass, while the corpus uteri looks like a mere appendix. The cervix is not only twice as long as the body, but its walls are much thicker. The vaginal portion and the os externum are disproportionately larger, and of quite irregular figure. The plicm of the cervical portion are continued almost to the fundus uteri, and there is no sharp dividing line between the two parts by an internal os. Not infrequently the small and thin-walled uterine body lies ante flexed upon the anterior surface of the cervix.
While all the other organs seem to develop during the period of child hood, the genitals do not. As puberty approaches there sets in not only a rapid growth, but also a change of form. The cervix does not develop as does the body of the organ. The vaginal portion becomes thinner, and the external os becomes relatively narrower, and assumes its proper round or oval form. The uterine body increases until finally it is twice as large as the cervix. Its walls become thicker than those of the latter organ, till finally the cervix appears as a mere appendage of the uterus. Gradually the folds of the uterine cavity disappear, and the developed os internum forms a sharp boundary between the uterine and the cervical canal.
These changes are not all complete by the time the menses begin; conception and birth may occur without the uterus having reached its full development. This seems to occur by about the twentieth year.
This peculiar post-foetal uterine development is important for the un derstanding of certain anomalies and malformations of that organ.
For the purpose of simplifying our ideas of the origin of the various developmental faults of the uterus, L. Furst has divided the entire period into five stages: The first period is from the time of impregnation to the fifth week, and includes the primitive state of the organs, until the time when the changes in the sexual glands and the commencing atrophy of the Mfillerian ducts in the male and the Wolffian bodies in the female show the beginnings of sexual differentiation. The atrophy of the septum which divides the adjacent Miillerian ducts has not yet occurred. There is a simple cloaca, into which the intestine and the urachus (allantois) open. At the sixth week the sexual tubercles and folds, and at the eighth week the sexual hirrow, have formed, but the parts are as yet sexually indifferent.
The second period extends from the eighth to the twelfth week of fcetal life, at which latter period the septum ha.s entirely disappeared. The point of union of the Miillerian ducts has extended upwards. The cornu of the uterus is differentiated from the tube, and the point of their union is marked by the insertion of the lig. ut. rotundum. During the second half of this period the cloaca divides by means of the formation of a septum into the anal and urogenital openings.
The third period extends from the twelfth to the twentieth week. The depression at the fundus and the uterine horns have disappeared; the arbor vitm of the womb is formed, while the vagina is yet smooth. The orificium externum is formed. The perineum has become larger. While the vagina has increased in size, the sinus urogeuitalis has re mained behind, so that the bladder appears now to open into the genital canal, instead of the latter being subsidiary to it. The sinus urogenitalis has become the vestibulum vagime; the sexual tubercle has atrophied into the clitoris; the sexual folds have developed into the labia majors, and the internal edges of the genital furrow into the labia minora. The perineum has grown, and the hymen has been formed.
The fourth period is from the twentieth week to the end of fcetal life; and the changes occurring are not so marked as in the preceding periods. The vagina and the uterine and cervical mucous membrane become folded. The development of the vault of the fundus is characteristic of this period.
The fifth period extends from birth to puberty, and is marked by in crease in the thickness of the walls of the womb. Up to the sixth year the uterine mucous membrane gradually becomes smoother and smoother, till finally only a longitudinal fold is left.