THE MALE REPRODUCTIVE ORGANS
tissues of the pelvic region, although this may not be apparent to an observer. The considerable number in most litters, distributed between the two horns, suggests that should fetuses from both sides arrive together at the entrance to the body they might have to jostle for priority of passage.
The risk of collision is prevented by the arrangement of the circular muscle of the uterus, which is able to close the exit from one horn while simultaneously securing maximal enlargement of the exit from the other. The mechanism is so effective that a hand exploring the interior of the uterus in these circumstances is unable to locate the entry to the horn that is temporarily shut off. The arrangement does not operate at all times; both horns open freely into the body of the atonic uterus, which allows fetuses to be transferred from one horn to the other at cesarean section.After the first piglets have been expelled, those remaining may travel more freely possibly because they are now able to move through the lubricated tube provided by a succession of already-vacated embryonic membrane sacs and because the umbilical cords provide rather loose tethers. They may shift quite far and even slip past their neighbors while still attached to their placentas.
Some criteria that may be used for estimating the age of pig fetuses are provided in Table 35-1.