The vagina is a dilatable canal, extending from the uterus to the vestibule. Cranially, the vagina is limited by the fornix, which extends ventral to the cervix (). The cervix may protrude 0.5 to 1 cm into the vagina, and is 0.8 cm in diameter. The fornix is the deepest part of the vagina and lies ventral and cranial to the cervix. The length of the dorsal vaginal wall is less than that of the ventral wall because of the oblique position of the cervix. The vagina ends just cranial to the urethral opening. It is demarcated from the vestibule by a transverse mucosal ridge that extends 1 cm dorsally on each side of the midventral line. No definite hymen is present at this point in the bitch, although its vestige may sometimes be found at the vaginovestibular junction. In a 25-pound dog, the vagina averages 12 cm long and 1.5 cm in diameter. Both the length and diameter of the vagina increase considerably during pregnancy and during parturition. The longitudinal folds (rugae) of the vaginal mucosa are high, allowing for great expansion in diameter (). Smaller transverse folds connecting the longitudinal folds permit craniocaudal stretching of the vagina.
The cranial portion of the vagina within the pelvic cavity is covered dorsally by peritoneum that reflects onto the colon, forming the rectogenital pouch (). Ventrally, this portion of the vagina has a peritoneal covering that reflects onto the bladder, forming the vesicogenital pouch. Laterally, the dorsal and ventral peritoneal coverings of the vagina fuse and become part of the broad ligaments. The caudal half of the vagina is retroperitoneal, being connected dorsally to the rectum and ventrally to the urethra by means of loose connective tissue. Laterally, the caudal part of the vagina is related to the vaginal blood vessels and nerves and to the ureters. The right and left ureters, with their peritoneal coverings, cross the lateral surface of the uterovaginal junction. The portion of the vagina that is located retroperitoneally depends to a large extent on the fullness of the bladder and rectum.
The vaginal walls are made up of an inner mucosal layer, a middle smooth muscle layer, and an external coat of connective tissue and peritoneum (cranially). The tunica mucosa is non-glandular, stratified squamous epithelium. The epithelium changes in appearance during the various stages of the estrous cycle as can be seen with a laparoscope and confirmed with vaginal smears (). The tunica muscularis is composed of a very thin inner layer of longitudinal muscle, a thick circular layer, and a thin outer longitudinal layer. The inner longitudinal and circular layers encircle the external uterine orifice. The outer longitudinal layer blends with the muscular layer of the body of the uterus. The submucous tissue contains a rich plexus of blood vessels. On the caudoventral wall of the vagina, vestigial ductus deferens (ducts of Gartner), vestigial remains of the caudal portion of the embryonic mesonephric duct, are usually absent.
Vagina: Vessels and Nerves
Arterial blood is supplied to the vagina via the vaginal artery (formerly urogenital), a branch of the internal pudendal. In addition to its vaginal distribution, the artery also supplies branches to the bladder, urethra and vestibule. Its urethral branches anastomose with the caudal vesical artery. The vaginal veins are satellites of the arteries and drain into the internal pudendal veins. The lymphatics drain into the medial iliac lymph nodes. The vagina is innervated by sympathetic and parasympathetic nerves from the pelvic plexus and by sensory afferent fibers via the pudendal nerve.