The Form of the Pregnant Uterus 375 



more distensible and the longitudinal folds of mucous membrane 

 begin to disappear. Finally, when labor begins, the os, under 

 normal conditions, dilates until it becomes completely effaced 

 and is of the same dimensions as the vagina and uterus, so that 

 the two cavities now become continuous, with no distinct line of 

 demarcation between the two portions. After parturition has 

 occurred, the os and cervix normally resume their previous 

 condition very promptly, so that within a few days these parts 

 are approximately the same as before impregnation. 



The situation of the uterus is modified by the changes taking 

 place in its volume. At the same time, its growth must alter 

 in some degree the situation of other floating viscera. The 

 gravid uterus possesses the highest specific gravity among the 

 floating organs of the abdominal cavity and, as a consequence, 

 in our quadrupedal animals it soon descends to and rests upon the 

 abdominal floor. Its position upon the abdominal floor is slightly 

 modified in some cases by neighboring organs, when they are 

 sufficiently voluminous and possess a high specific gravity, like 

 the rumen of ruminants and the great colon of solipeds. As a 

 result, in the mare the pelvic flexure of the colon is displaced 

 somewhat to the right and the uterus passes along beneath it 

 to the left of the median line. In ruminants, on the other hand, 

 the uterus becomes slightly displaced to the right by the enormous 

 rumen. In the multiparous animals, none of which have any 

 very voluminous floating viscera, the gravid uterus becomes the 

 most important abdominal organ and takes first place along the 

 median line, displacing the other viscera to either side or upward. 



These changes come about somewhat slowly. At first the 

 gravid uterus rests partly in the pelvis and well suspended by 

 the broad ligaments in the sublumbar region, but, as soon as the 

 fetus has acquired any great volume, it at once bears the uterus 

 downward and forward onto the abdominal floor . and finally 

 pushes its way along this, as on an inclined plane, until it reaches 

 the diaphragm, where the most anterior extremity of the gravid 

 uterus Hes in close relation with the stomach, liver and dia- 

 phragm. 



During this change in the position of the uterus, the os uteri 

 is for a time dragged forward so that it is farther from the vulva 

 than in the non-pregnant animal, but later, in unipara, when the 



