The female reproductive system is built around the paired ovaries — elliptical structures with an outer germinal epithelium, a tunica albuginea, and a stroma split into a follicle-bearing outer cortex and a vascular inner medulla — held in place by the mesovarium ligament. A pair of fallopian tubes (oviducts) run from each ovary to the uterus, starting with a fringed, funnel-shaped infundibulum that sweeps up the ovum after ovulation, widening into the ampulla, and narrowing into the isthmus before joining the uterus. The uterus is a thick-walled, muscular, pear-shaped organ with three tissue layers — the outer perimetrium, the contractile myometrium, and the inner, cyclically-changing endometrium — narrowing below into the cervix, which opens into the vagina; together the cervical canal and vagina form the birth canal. External to the vagina lies the vulva: the labia majora and minora, the clitoris, and the hymen (a thin membrane that can be torn by intercourse or unrelated activity, so it is not a reliable indicator of virginity), supported by the lubricating Bartholin's glands (homologous to the male bulbourethral glands) and Skene's glands (homologous to the prostate). The mammary glands, modified sweat glands functional only in females, are built from lobes of milk-secreting alveoli that drain via mammary ducts, a mammary ampulla, and a lactiferous duct to the nipple, and develop from puberty onward under hormonal control.