Sherwin B. Nuland in The New Republic:
As far back as has been recorded of the history of human societies, men have equated life with movement. If our ancestors of antiquity could feel stirrings inside their bodies, it must mean that large, living structures were at the very least shifting their positions, and perhaps even migrating from place to place within the mysterious recesses of the internal cavities that encompassed them, whether the abdomen or the chest. Like other peoples of their time, for example, the Egyptians believed that the inner organs were so many distinctive and individual creatures, with whims and moods of their own that determined their peregrinations from neck to pelvis.
Of all the named structures within the abdomen and the chest, those associated with reproduction retained the mysteries of their willful behavior long after others had been solved to the satisfaction of physicians and philosophers. When, in his Timaeus, Hippocrates's contemporary Plato called the uterus “an animal within an animal,” he meant it to be taken literally. He was echoing a common belief of his time and earlier when he stated that under proper conditions the womb — or hystera, as it was called in Greek — “becomes seriously angry and moves all over the body.” Chief among those proper conditions was any frustration of its desire to bear children.