The vestibule forms from the urogenital sinus and, as mentioned previously, it is separated from the vagina initially by the hymen. The vestibule develops as a structure separate from the vagina and fuses with it later in development. The vestibule, however, is similar in structure to the vagina and is lined by stratified squamous epithelium. Below the lamina propria of this structure is a tunica muscularis with an inner circular and outer longitudinal smooth muscle layer. The outer muscular layer is composed of two different muscles, the constrictor vestibule and the constrictor vulvae.The vestibular glands are tubuloalveolar glands present in the constrictor vestibule muscle and are homologous to the male bulbourethral glands and the urethral glands.
The clitoris is homologous to the penis. It is composed of the body, glans and preputial covering. The body is composed of cavernous tissue, smooth muscle and adipose tissue surrounded by a thick collagenous capsule. The glans is composed of cavernous tissue (in canines and horses) or highly vascular connective tissue. The preputial covering is the reflection of the stratified squamous epithelial lined vestibular mucosa.
The labia are simply folds of skin just caudal to the opening of the vestibule. The histologic structure is similar to what you will read in the integumentary chapter.