(the outer uterine layer).

The myometrium can be divided into three layers:
* The inner one-third thickness is termed the junctional or sub-endometrial layer. In most mammals it is characterized by fibers oriented in a circular way. Humans, having a single uterus from the fusion of two Müllerian ducts, have the fibers forming two cones. Mice have an unfused uterus, so the arrangement is simply circular.
* The middle layer occurs in both mice and humans. In mice it is a hard-to-spot