Vagina and Vestibule - Anatomy & Physiology

Introduction

The vagina constitutes the part of the female reproductive tract between the cervix and the vulva. With the vestibule and vulva, it is the copulatory organ and the birth canal. The hymen is the poorly developed, vestigial, mucosal folds at the junction of the vagina and vestibule.

Anatomical Boundaries

Vagina

• Extends from the external ostium of the uterus to the entrance of the urethra.
• Long and thin walled.
• Located in the median position within the pelvic cavity.
• Located between the rectum dorsally and bladder ventrally.
• Mostly retroperitoneal, but the cranial parts are covered by peritoneum.

Vaginal Fornix

• Protruding cervix restricts the lumen of the cranial vagina to a ring-like space known as the fornix.
• This is a cranioventral recess in the mare and bitch.
• The recess is caudoventral in the cow and ewe.

Vestibule

• Extends from the external urethral opening to the external vulva, so combines urinary and reproductive functions.
• Shorter than the vagina.
• Lies mostly behind the ischial arch, sloping ventrally to its opening at the vulva.
• The wall contains vestibular glands.
  • Secretions from these glands keeps the mucosa of the vestibule moist and facilitates coitus and parturition.
  • At oestrus, the odour of these secretions sexually stimulates the male.

Vestibular Bulbs

• Present in the mare and bitch
• Dark patches on the vestibular walls mark the position of vestibular bulbs.
• Vestibular bulbs are a concentration of veins forming erectile tissue, this is the female homologue of the bulb of the penis.
• During the dog's 'tie', the bulbs are erect and press against the penis, caudal to the enlarged bulb of the glans.

Vulva

• Formed by two labia that meet at dorsal and ventral commissures surrounding the vertical vulvar opening.
  • Dorsal commissure is rounded.
  • Ventral commissure is pointed.
  • In the mare the dorsal commissure is pointed and the ventral is rounded.

Clitoris

• Partial homologue of the penis.
• Located within the ventral commissure of the vulva.
• Can be divided into two parts:
  • Body (corpus)
  • Glans (glans clitoridis)
• Has left and right crura that attach to the ischiatic arch.
  • Crura come together to form the body.
• Lies within a fossa, largely covered by a mucosal fold which is the female equivalent of the prepuce.
• The glans is the only exposed part of the clitoris.

Os Clitoridis

• Found radiographically in some bitches.
• Homologue of the os penis

Histology

Vagina

• Fibromuscular canal
• The wall consists of:
  • Mucosal layer lined by stratified squamous epithelium
  • Layer of smooth muscle
    • Inner circular and outer longitudinal layers
  • Outer layer of adventitia
  • Not lined by Mesothelium
    • Merges with the adventitial layers of the bladder anteriorly and rectum posteriorly.
• This combination of elastic fibres and muscular layers allows the massive distension that occurs during parturition.
• After coitus, contraction of the smooth muscle layer ensures that the pool of semen remains within the cervical region.
• When relaxed, the vaginal wall collapses to obliterate the lumen and the vaginal epithelium is thrown into folds.
• The dense lamina propria contains:
  • Elastic fibres
  • Rich plexus of small veins
  • Aglandular

Vestibule

Vestibular Glands

• Cow, queen and sometimes the ewe have major vestibular glands ('glands of Bartholin').
  • In the cow and Ewe, a large glandular mass is present on each side of the vestibule. This drains via a single duct.
• Bitch, ewe, mare and some have only minor vestibular glands.
  • Vestibular glands are small in the bitch, but numerous. Duct openings are arranged in a linear series.

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