External Genitalia

By | July 27, 2013

The external genitalia of the female dog include the pudendum femininum (vulva), clitoris, and urethra feminina ().

External Genitalia: Pudendum Femininum

The pudendum femininum (vulva) lies caudal to the vestibule and consists of two lips, labii (labium pudendi [vulvae]) joined dorsally and ventrally by commissures (commissuura labiorum dorsalis et ventralis) and separated by a narrow cleft, the rima pudendi. The labia form the external boundary of the vulva and in part are homologous with the scrotum of the male. The labia are soft and pliable, being composed of fibrous and elastic connective tissue, striated muscle fibers (m. constrictor vulvae), and an abundance of fat. The vaginal processes, containing the round ligaments of the uterus, often end in the subcutaneous connective tissue of the labia. The distance between the dorsal commissure of the labia and the anus is 8 to 9 cm. The dorsal commissure lies at or slightly ventral to the dorsal plane passing through the symphysis pelvis. The ventral portions of the labia, with their uniting commissure, form a pointed projection extending ventrally and caudally from the body, usually with a tuft of hair.

External Genitalia: Clitoris

The clitoris (), the homologue of the male penis, is composed of paired roots (crura clitoridis), a body (corpus clitoridis), and a glans [glans clitoridis). The roots and body are homologues of the male corpora cavernosa penis, and the glans clitoridis is homologous with the glans penis, although it is not bipartite in structure. The body of the clitoris in the dog has both fatty and erectile tissue. It is covered by a tunica albuginea. The clitoris of the dog does not normally contain a bone but an os clitoridis can be present (). In the normal bitch, there are elongate masses of erectile tissue lying deep to the vestibular mucosa and united to each other dorsally by an isthmus. These are the vestibular bulbs [bulbus vestibult) that correspond to the bulb of the penis in the male (). The bulbs are each supplied by a terminal branch of the internal pudendal artery, homologous to the artery of the bulb in the male. The glans clitoridis, erectile in structure, is very small and projects into the fossa clitoridis (). The wall of the fossa is partially folded over the glans clitoridis dorsally. This fold corresponds to the male prepuce (preputium clitoridis). The free part of the clitoris (glans) is approximately 0.6 cm long and 0.2 cm in diameter in an average-sized dog; the distance from the ventral commissure of the vulva to the glans clitoridis is 2 to 3 cm, and that from the ventral commissure to the fundus of the fossa of the clitoris is 3 to 4 cm. The opening of the fossa is approximately 1 cm in diameter. Nitschke (1970) has described the structure of the clitoris and vagina in the dog. Lindsay (1983) has described and illustrated the endoscopic appearance of the vestibule, vulva, and vagina in the cyclic, noncyclic, and spayed bitch. Her endoscopic photographs, in color, show clearly that the changes in mucosal appearance at different phases of the estrous cycle can be recognized grossly.

An os clitoridis can develop in response to an altered hormone balance and may be more than 2.5 cm long. Grandage and Robertson (1971) reported an os clitoridis in a normal Welsh Corgi bitch that subsequently mated and whelped. The bone was seen radiographically to be 13 mm long, laterally compressed, and pointed at its apex.

Although it is said that the dog usually lacks an os clitoridis, it may only be that it is rarely looked for and rarely reported. In four cases that came to Evans’s attention, one was less than a centimeter long and three were large. The small one was presented because of a urinary problem, which was solved by the removal of the clitoris. Two of the large ones (a Kerry Blue Terrier with a 4-cm clitoris and 3.5-cm os () and a Shorthair German Pointer with a 6-cm clitoris and 5-cm os () were brought in by practitioners after surgical removal because of an inflamed clitoris. The fourth os was found in a dissection specimen. When an os clitoridis is present it may be associated with a natural endocrine disturbance or more probably is the result of androgen or progestin therapy. Shane et al. (1969) fed methyltestosterone (150 meg per kg body weight per day) to female Beagles starting on the day of first mating and continuing through pregnancy, parturition, and lactation, and for 9 months thereafter. A total of 87 pups were born to 14 females. Of these pups 42 were intersexes, and 45 were normal males. All of the intersex pups tested (nine) were genetic females. The intersex pups had a phallus that was partially hypospadic in some. Urine was voided through the phallus and the position assumed for urination was that of a female dog. No scrotum or vulva was present, but a bone was sometimes present in the phallus. On the basis of karyotype studies, the intersex pups were classified as female pseudohermaphrodites, and thus the phallus can be considered a hypertrophied clitoris, which in some cases has an os clitoridis. The development of the ovary and uterine tubes appeared to be normal in all female pups. When adult, some of these female pseudohermaphrodites ovulated, formed corpora lutea, and accumulated fluid in the uterus. The latter finding is consistent with progesterone secretion.

External Genitalia: Structure

Recognition of the location of the external urethral orifice is important for the purpose of catheterization. It is common for the fossa clitoridis to be mistaken for the urethral opening, and the catheterization attempt is unsuccessful. The urethra opens on a tubercle 4 to 5 cm cranial to the ventral commissure of the vulva at the level of the ischial arch.

The mucosal surface of the vulva is covered by stratified squamous epithelium. A variable number of lymph nodules may cause prominences to appear on the mucosa. Small minor vestibular glands, lobular in structure, open ventrally on each side of the median ridge connected to the urethral tubercle. These mucosal glands are located deep to the vestibular smooth muscles and the striated constrictor vestibuli muscles. The body of the clitoris consists of fat, elastic connective tissue, and a peripheral tunica albuginea. The glans clitoridis, made up of erectile tissue, contains numerous sensory nerve endings. The vestibular bulbs are also composed of cavernous tissue. The labia, covered with stratified squamous epithelium, are rich in sebaceous and tubular glands and also contain fat, elastic tissue, and smooth and striated muscle fibers.

External Genitalia: Muscles

In addition to the usual unstriped muscle fibers, similar to those of the vagina, the vestibule and vulva each possesses a striated circular muscle (). The most cranial of the two is the larger vestibular constrictor muscle (m. constrictor vestibuli). It is incomplete on the dorsal surface of the vestibule, but fuses along its dorsocaudal border to the external sphincter of the anus. Its fibers run diagonally in a cranioventral direction, encircling the urethra, vestibule, and caudal portion of the vagina before it joins its fellow of the opposite side. It constricts the vestibule.

Immediately caudal to the m. constrictor vestibuli is the relatively thin constrictor vulvae muscle (m. constrictor vulvae), which is the muscle of the labia. This muscle is continuous dorsally with the external anal sphincter, arising from the caudal fascia ventral to the first and second caudal vertebrae, and encircling the vulva and vestibule approximately 1 cm caudal to the point where the urethra enters the genital tract. The constrictor vulvae muscle blends with the vestibular constrictor to a slight degree. The vulvar constrictors fuse together below the vulva, cranial to the ventral commissure. They lift the labia dorsally prior to intromission of the penis, allowing it to enter the vagina more easily. The vestibular and vulvar constrictors together are homologous with the bulbospongiosus muscles of the male. They lie superficial to the vestibular bulbs. Their counterparts in the male are peripheral to the bulb of the penis. The ischiourethralis muscles () arise from the caudomedial surface of the tuber ischii, on each side, and insert upon the poorly developed central tendon of the perineum.

The ischiocavernosus muscles () are small in the female. They arise bilaterally from the caudal edge of the ischium and attach to the crura clitoridis. This is similar to the manner in which they insert upon the corpora cavernosa in the male.

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