the vagina is the pathway that serves as a dual passageway for both the menstrual flow and child birth. additionally it receives sperm from the male penis during copulation, it is a fibromuscula organ lined mucus membranes and in a 25 pound dog it measures roughly 12cm long, 1.5cm in diameter in a human 10cm in length, both the length and diameter will increase during pregnancy and birth. the vagina is made up of three layers a inner mucosal layer, a middle layer of smooth muscle and a outer coat of connective tissue, the mucosal layer is made up of many folds, these folds allow the vagina to stretch during pregnancy (Wells, 2011)
Urogenital sinus
the urogenital sinus is the part of the reproductive system that separates the urethra from the vagina, the urethra and the vagina at the lower section unite to form a common pathway and opens to the exterior of the body,. in a human the urogenital sinus is separate from the vagina through out its length and the vestibule is a much reduced area
(Anon., n.d.) the image is of a kangaroos reproductive tract, it clearly shows the Urogenital sinus