LORI.equine

The corpus cavernosum clitoridis in mares

Keywords: corpus, cavernosum, clitoris, vulva, mare, equine

The clitoris is of course, the female homolog of the penis. Indeed, as reviewed by Puppo in 2013 in humans, it has all the main structures identifiable in the penis, including a corpus cavernosum arising from two crura and a corpus spongiosum that is continuous with the glans clitoridis. Indeed, during sexual arousal, the corpora cavernosum becomes turgid like that of the male. Although the clitoris is present in all domestic animals, it is only in rabbits that it has been well studied, serving as a model for pharmacotherapy in humans. Apart from its sinuses and gross external anatomy, the clitoris remains poorly described in mares.

Females are the default sex. It is only because of the presence of the testicles and modification of the hypothalamic-pituitary axis early in embryonic development that males develop. Consider then how the penis is modified from its precursor; the clitoris. In essence, the penis can be looked upon as an extended clitoris. It contains a urethra that has extended beyond the external urethral orifice present in females. In normal males, the urethra extends and closes on its dorsal and  caudal surfaces then (as it curves down and between the hind legs) its ventral surface. It is only in males with hypospadias, that this closure is incomplete. In embryonic males, the penis grows caudal then ventral and in most animals (less so in felids) cranially between the hind limbs.

Upon dissecting the lateral wall of the vestibule in mares, the following structures become visible. See figure 1.

Figure 1. The constrictor vestibuli muscle bulges at left (see yellow arrows) due to the presence of a cavernous plexus between that muscle and the vestibule itself.  The black arrows indicate the dorsal commissure of the vulva lips and the ventral extent of the clitoris. Once the constrictor vestibuli muscle is transected and reflected as seen in the image at right, a vascular plexus (ringed yellow) is revealed.  In the image at right, the ventral commisure of the vulva has been rotated so that the location of the plexus, relative to the clitoris can be appreciated. Image size: 2089 x 1653 px

This vascular plexus is now evident as the corpus cavernosum clitoridis. It can be isolated from the wall of the vestibule as seen in figure 2.

Figure 2. A pair of scissors has been inserted through the fascia, medial to the corpus cavernosum, isolating it from its adnexa. On the right side, the corpus cavernosum has been transected to show its proximal and distal extremities and its cavernous nature. The plexus slants ventrally from the ischial arch (not shown) to the base of the clitoris.  Image size: 1300 x 1285 px

Perhaps obviously, a corpus cavernosum clitoridis is found on either side of the vestibule.

Figure 3. This image shows the cut surface of the corpus cavernosum submerged in water. It has been transected using a V-shaped incision with the open section of the V closest to the observer. Image size: 4686 x 2704 px

A further dissection is underway to demonstrate the proximal attachment of the corpus cavernosum clitoridis to the ischial arch, the attachment of its counterpart in males. That dissection will also show how the corpus cavernosum is continuous with the clitoris in mares. The counterpart to the corpus spongiosum urethrae in human males is also present in the clitoris in human females. There is little doubt in the author's mind that situation can be demonstrated in mares as well.

Veterinary anatomy and physiology texts pay little attention to the clitoris. It is of course, known as a site for site for the isolation of Taylorella equigenitalis and a (not infallible) sign of sexual receptivity as the clitoris exposes itself rhythmically during teasing (winking).  But other than this, is the clitoris merely a vestigial organ in the mare or does it play an important role in fertility? Veterinary literature appears to be devoid of this information. Such studies may be valuable because clitoral stimulation it is known to shorten the time from the onset of estrus to ovulation in cattle. Clitoral stimulation also increased conception rates in some studies when artificial insemination was used in beef cows. It also plays an important role in human sexuality and is in fact, a target organ for sildenafil (Viagra®) treatment in women as well as men.

Selected references: 

Lunstra, D. D. et al. 1985. Increasing pregnancy rate in beef cattle by clitoral massage during artificial insemination. Digital commons report. U. Nebraska. https://tinyurl.com/y7vl8jjo

McAllister, R.A. and Sack, W.O. 1990. Identification of anatomic features of the equine clitoris as potential growth sites for Taylorella equigenitalis.J Am Vet Med Assoc. 196:1965-6

Park, K. et al. 1998. Sildenafil Inhibits Phosphodiesterase Type 5 in Human Clitoral Corpus Cavernosum Smooth Muscle. Biochem. biophysical. Res. and Comm. 249: 612 – 617

Puppo, V. 2013. Anatomy and physiology of the clitoris, vestibular bulbs, and labia minora with a review of the female orgasm and the prevention of female sexual dysfunction. Clin. Anat. 26:134–152

Segura, C.V.M. and Rodriquez, R.O.L. 1994. Effect of clitoral stimulation after artificial insemination on conception in Zebu-crossbred heifers in the tropics. Theriogenology 42:781-787

Spani, F. et al. 2017. A sexual selection framework in Primates: raising ossa genitalia again. Poster presentation.  Roma, Congresso della Società Italiana di Biologia Evoluzionistica, Dipartimento di Scienze, Università degli Studi Roma Tre.

Vemulapalli,S. and Kurowski, S. 2000. Sildenafil relaxes rabbit clitoral corpus cavernosum. Life Sciences 67:23–29