The uterus is divided into layers, each with its own structural and functional characteristics. The mucosal layer, or endometrium, the muscularis layer, or myometrium, and the serosal layer, or perimetrium are the most basic classifications of these layers. The endometrium is composed of two layers: the stratum functionalis and the stratum basalis. The stratum functionalis expands and vascularizes during the menstrual cycle and is then sloughed off during the menstrual process, whereas the stratum basalis remains relatively constant. The myometrium allows the uterine cavity to expand and contract and responds to the hormone oxytocin.