THE VESTIBULE AND VULVA
The dorsal wall of the vestibule only gradually departs from the line of the rectum and anal canal; the longer ventral wall slopes more steeply downward beyond the ischial arch (Figure 22-8/9).
Noteworthy features are the urethral opening at the cranial limit and the clitoris within the ventral commissure of the vulva. The clitoris varies much in development and is largely covered by a transverse preputial fold that attaches to the dorsal surface of its glans (Figure 22-13, C/12). The fold and ventral commissure together constitute the prepuce. The clitoris is very prominent in mares in heat when exposed by “winking” movements of the labia. Laterally and ventrally it is separated from the labia by a clitoral fossa. Several sinuses of varying depth invade the glans. These may harbor the organism responsible for contagious equine metritis. Further mucosal recesses are present in the ventral parts of the clitoral fossa and labia. Although no major vestibular glands exist, numerous minor glands discharge within small depressions, ranked in ventral and dorsolateral rows. The mucosa overlying the vestibular bulb, situated in the lateral wall toward the vulva, is more darkly colored.The vulva is unusual in having rounded ventral and pointed dorsal commissures, which is a reversal of the usual arrangement (Figure 22-14/3). The relationship of the vulva to the pelvic skeleton varies considerably. Usually it is largely ventral to the pelvic floor with the cleft closed. Sometimes, and quite commonly in Thoroughbreds, the opening is more dorsal and closure is less effective; in this circumstance, air may be drawn into or expelled from the tract with each change in intraabdominal pressure. Bacteria may be introduced, and the contamination may spread to the endometrium, which may result in sterility. The same fault (wind-sucking) may be due to laceration of the vulva at a previous parturition.