Vagina

The vagina
(From Latin vagĭna, literally "sheath" or "scabbard") is a fibromuscular tubular tract leading from the uterus to the exterior of the body in female placental mammals and marsupials, or to the cloaca in female birds, monotremes, and some reptiles. Female insects and other invertebrates also have a vagina, which is the terminal part of the oviduct. The Latinate plural "vaginae" israrely used in English.
The word vagina is quite often used colloquially to refer to the vulva or female genitals generally; technically speaking, the vagina is a specific internal structure. In humans, the passage leads from the opening of the vulva to the uterus (womb). It lies midway between the anal tract and the urethra

Location and structure
The human vagina is an elastic muscular canalthat extends from the cervix to the vulva.
Although there is wide anatomical variation, the length of the unaroused vagina of a woman of child-bearing age is approximately 6 to 7.5 cm (2.5 to 3 in) across the anterior wall (front), and 9 cm (3.5 in) long across the posterior wall (rear). During sexual arousal the vagina expands in both length and width. Its elasticity allows it to stretch duringsexual intercourse and during birth to offspring. The vagina connects the superficial vulva to the cervix of the deep uterus.
If the woman stands upright, the vaginal tube points in an upward-backward direction and forms an angle of slightly more than 45 degrees with the uterus. The vaginal opening is at the caudal end of the vulva, behind the opening of the urethra. The upper one-fourth of thevagina is separated from the rectum by the recto-uterine pouch. Above the vagina is the Mons pubis. The vagina, along with the inside of the vulva, is reddish pink in color, as are most healthy internal mucous membranes in mammals. A series of ridges produced by folding of the wall of the outer third of the vagina is called the vaginal rugae. They are transverse epithelial ridges and their functionis to provide the vagina with increased surface area for extension and stretching.
Vaginal lubrication is provided by the Bartholin's glands near the vaginal opening and the cervix. The membrane of the vaginal wall also produces moisture, although it does not contain any glands. Before and during ovulation, the cervix's mucus glands secretes different variations of mucus, which provides analkaline environment in the vaginal canal that is favorable to the survival of sperm.
The hymen is a membrane of tissue which is situated at the opening of the vagina. As in many female animals, the hymen covers the opening of the vagina from birth until it is ruptured during sexual or other activity. The tissue may be ruptured by vaginal penetration, delivery, a pelvic examination, injury, or sports.The absence of a hymen does not indicate prior sexual activity, as it is not always ruptured during sexual intercourse.] Similarly, its presence does not indicate a lack of prior sexual activity, as light activity may not rupture it, and it can be surgically restored.

Function
The vagina has several biological functions.
Sexual activity
Further information: Human sexual activity
Theconcentration of the nerve endings that lie close to the entrance of a woman's vagina can provide pleasurable sensation during sexual activity, when stimulated in a way that the particular woman enjoys. During sexual arousal, and particularly the stimulation of the clitoris, the walls of the vagina self-lubricate. This reduces friction that can be caused by various sexual activities. Research has foundthat portions of the clitoris extend into the vulva and vagina.[7]
With arousal, the vagina lengthens rapidly to an average of about 4 in.(10 cm), but can continue to lengthen in response to pressure.[8] As the woman becomes fully aroused, the vagina tents (last ²⁄₃) expands in length and width, while the cervix retracts.[9] The walls of the vagina are composed of soft elastic folds of mucous...