sex life. Anatomy and stimulation The abundance of nerve endings in the anal region and rectum can make anal sex pleasurable for men or women. The internal and external sphincter muscles control the opening and closing of the anus; these muscles, which are sensitive membranes made up of many nerve endings, facilitate pleasure or pain during anal sex. Human Sexuality: An Encyclopedia states that "the inner third of the anal canal is less sensitive to touch than the outer two-thirds, but is more sensitive to pressure" and that "the rectum is a curved tube about eight or nine inches long and has the capacity, like the anus, to expand". Research indicates that anal sex occurs significantly less frequently than other sexual behaviors, but its association with dominance and submission, as well as taboo, makes it an appealing stimulus to people of all sexual orientations. In addition to sexual penetration by the penis, people may use sex toys such as a dildo, a butt plug or anal beads, engage in fingering, anilingus, pegging, anal masturbation or fisting for anal sexual activity, and different sex positions may also be included. Fisting is the least practiced of the activities, partly because it is uncommon that people can relax enough to accommodate an object as big as a fist being inserted into the anus. In a male receptive partner, being anally penetrated can produce a pleasurable sensation due to the object of insertion rubbing or brushing against the prostate through the anal wall. This can result in pleasurable sensations and can lead to an orgasm in some cases. Prostate stimulation can produce a deeper orgasm, sometimes described by men as more widespread and intense, longer-lasting, and allowing for greater feelings of ecstasy than orgasm elicited by penile stimulation only. The prostate is located next to the rectum and is the larger, more developed male homologue (variation) to the female Skene's glands. It is also typical for a man to not reach orgasm as a receptive partner solely from anal sex. General statistics indicate that 70–80% of women require direct clitoral stimulation to achieve orgasm. The vaginal walls contain significantly fewer nerve endings than the clitoris (which has many nerve endings specifically intended for orgasm), and therefore intense sexual pleasure, including orgasm, from vaginal sexual stimulation is less likely to occur than from direct clitoral stimulation in the majority of women. The clitoris is composed