Vagina

Ventral view, pelvic region

The vagina is a structure unique to females. It is an accessory sex organ in that it receives the penis during intercourse and is where semen is normally deposited. Part of it is derived from the urogenital sinus of the embryo. It is described by many authors as a potential space and is approximately 3 inches in length. Its walls have rugae that help in expansion without a huge increase in pressure during childbirth. It is an excretion duct for the uterus as well as being part of the birth canal. The inferior half is in the perineum, while the superior half is in the pelvis. The epithelial cells lining the vagina release glycogen into the lumen. This carbohydrate undergoes bacterial fermentation which produces lactic acid. The lactic acid discourages harmful bacteria but also is harmful to spermatozoa. Note that the probe on your right is inserted into the urethra. There is a major difference between the vagina of a human and the vagina of a cat. In the human the vagina exits separately to the outside from the urethra. In a cat the vagina and the urethra meet to form an internal vestibule, which is also called a urogenital sinus. The vestibule of an adult human is external. The probe on the left is pointing at the caudal end of the vagina. The vagina extends from that point cranially to the cervix.