The external female genitalia consist of the outer (majora) and inner (minora) labia containing erectile tissue that surround the vaginal introitus. Normally the outer labia meet and cover the introitus, but in some women the inner labia protrude even when they are sexually unaroused. Sexual arousal creates vasocongestion especially in the labia minora which protrude through the majora adding approximately 1 to 2 cm to the length of the vagina. The labia minora become erotically sensitive to touch and friction when engorged.
Although the clitoris is the homologue of the penis, its precise anatomical structure is still uncertain. The most recent description by O'Connell et al.is of a triplanar complex of erectile tissue with a midline shaft lying in the medial sagittal plane about 2 to 4 cm long and 1 to 2 cm wide, which bifurcates internally into paired curved crura 5 to 9 cm long, and externally is capped with a glans about 20 to30 mm long with a similar diameter. Two vaginal bulbs of erectile tissue are closely applied on either side of the urethra. The shaft's erectile tissue consists of two corpora cavernosa surrounded by a fibrous sheath (tunica albuginea) and the whole is covered by a clitoral hood formed in part by the fusing of the two labia minora. The uncertainty concerns the location and extent of the female corpus spongiosum. Some describe it as wrapped around the urethra, others state that the vaginal (vestibular) bulbs on either side of the vaginal wall are spongiosus tissue and unite ventrally to the urethral meatus to form a thin strand of erectile tissue ending in the glans. Recent studies by van Turnhout et al.(i3) have clarified the situation. They confirmed by dissections in fresh cadavers that the bilateral vestibular bulbs terminate into the glans clitoridis.
With sexual arousal, the blood flow to the clitoris is increased probably by a mechanism involving its vipergic (VIP) and nitrinergic (nitric oxide) innervation leading to its tumescence (swelling) but, contrary to many inaccurate descriptions, without true erection (i.e. without rigidity). The enhancement of its blood flow is paralleled by an increased sensitivity to touch and friction especially of the glans.
There is a triangular area of mucous membrane that surrounds the urethral meatus, extending from just below the glans of the clitoris to the entrance of the vagina. This area has been called the 'periurethral glans'(30) and is thought to be capable of erotic stimulation, especially during penile thrusting, as it is known that the tissue is pushed and pulled into the vagina by the movement.(30) The periurethral glans is actually part of the corpus spongiosum if we accept the findings of van Turnhout et al.(43) which suggest that it is the homologue of the the male glans. The extent, mobility, density of innervation, and sensitivity of this erotic site may explain why some women find it easy to reach orgasm during penile thrusting alone.
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