The vagina is an elastic canal for birth, sex, and menses
In female mammals the vagina is a muscular, stretchy canal from vulval vestibule to cervix. In humans a hymen often partly covers the introitus; the same passage carries menstrual flow in humans and close primate relatives and enables copulation and birth.
During intercourse, vaginal walls lubricate and create friction that can help drive ejaculation and fertilization. Pleasure and bonding coexist with risks of sexually transmitted infections, mitigated by safer-sex practices; other local health issues also occur. Cultural meanings have been intense across societies even as the anatomy itself is a straightforward reproductive conduit.
The name comes from Latin vāgīna, a sheath or scabbard, and in pregnancy the organ is also called the birth canal. Everyday speech often stretches the word to cover the vulva, a slip that can cause medical or legal confusion: one clinical description runs from the hymen or its remnants to the cervix, while a legal one starts at the vulva between the labia.
At rest the canal is collapsed, its front and back walls touching while stiffer side walls give it an H-shaped cross section. Because the cervix sits in the front wall, that wall measures about 7.5 centimetres against roughly 9 for the back, and both lengthen and widen during arousal. The wall has a non-keratinized stratified squamous lining over a vessel-rich lamina propria, then smooth muscle with circular fibres inside longitudinal ones, then an outer adventitia. That muscle contracts only weakly; the heavy pushing of childbirth comes from pelvic floor muscles attached around it.
Its embryonic origins are still debated. The majority view, Koff's 1933 account, derives the upper two-thirds from the Müllerian ducts and the lower part from the urogenital sinus, while Bulmer in 1957 traced the lining entirely to the sinus.
Source: Vagina
