Cyclopædia of obstetrics and gynecology . projection at the place where the middleruns into the lower third of the canal; and this gradually grows inwards. GENERAL SKETCH OF UTERINE DEVELOPMENT. 183 and encroaches upon the lumen of tlic canal. In the last months of preg-nancy this ring undergoes further changes and becomes the portio vagi-nalis and the os externum. A little later the internal os is formed in thesame way^ and then the walls of the canal between the two rings beginto become thicker and folded, and the cervical canal is formed. Thefundus uteri is later formed by thickening of the
Cyclopædia of obstetrics and gynecology . projection at the place where the middleruns into the lower third of the canal; and this gradually grows inwards. GENERAL SKETCH OF UTERINE DEVELOPMENT. 183 and encroaches upon the lumen of tlic canal. In the last months of preg-nancy this ring undergoes further changes and becomes the portio vagi-nalis and the os externum. A little later the internal os is formed in thesame way^ and then the walls of the canal between the two rings beginto become thicker and folded, and the cervical canal is formed. Thefundus uteri is later formed by thickening of the wall between the tubes;the canal is changed into a triangular cavity, and the uterine cavity isformed. In the meantime the lower vaginal third of the genital canal has be-come dilated and folded upon itself. The margins of the canal, whichproject first into the cloaca and then into the sinus urogenitalis, are trans-formed into the hymeneal ring, thenceforward forming the sharp dividingline between the external and the internal Fig. 11.—a, Uterine body; b, external genitals; c, tubes; d, vagina; e, portio; /, hymen; g, cer-vix uteri; h, fundus uteri; i, os internum. We have seen that the middle third of the genital canal becomeschanged 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 to man and the quadrumana; in all the lower animals agreater or less division remains. Both Miillerian ducts may remain en-tirely separated, or, if united at their external orifices, may form twocomplete uteri and vaginae. This occurs with many rodents, as in thehares. In other animals not only the vagina, but the lower part of theuterus also, is single, while the greater portion is divided into two separatehorns. This is the case with the mouse. In others, as in the carnivora,there is a common uterine cavity, whil
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