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Magnetic resonance imaging of male and female genitals during coitus and female sexual arousal

The study was conducted to see if magnetic resonance imaging could establish whether previous assumptions about the nature of the interaction of penis and vagina during sexual intercourse were correct.

To summarize these findings, the resonance images obtained during intercourse showed that in the missionary position the penis is shaped rather like a boomerang and about one third of the length of the penis consists of the root within the body.

During female sexual arousal where the woman was not engaged in intercourse, the uterus was raised and the length of the vaginal barrel increased.

However, the size of the uterus did not apparently increase during the period of sexual arousal.

Masters and Johnson researched these issues in the 1960s using an artificial penis that was designed to imitate natural intercourse, as well as by direct observation through a speculum and the use of palpation.

Video - MRI scan of sexual intercourse

They pioneered research in this field and discovered that during sexual arousal in a woman, the vaginal wall underwent a process known as the vaginal tenting, where the anterior wall of the vagina moved backwards and upwards.

They also claimed that the uterus increased in volume by between 50 and 100% during sexual arousal, an increase which disappeared between 10 and 20 minutes after orgasm.

If there was no orgasm, the size of the uterus returned to normal in about an hour; Masters and Johnson assumed that the increase in volume of the uterus was due to engorgement with blood during sexual arousal.

At the time of this study, Magnetic resonance imaging had already been used as a diagnostic tool in the study of erectile dysfunction.

It was therefore not a large intuitive leap to imagine using the same technology to investigate the shape and orientation of the male and female genitals during sexual intercourse.

The participants were heterosexual couples over the age of 18 years with normal genitals and a small to average ratio of weight to height.

Each couple had sexual intercourse out of sight of the investigators and within the central tube of the magnetic resonance imaging equipment.

During intercourse couples were asked to lie with their pelvic region near a marked position in the tube and not move as the imaging took place.

Using a rapid imaging sequence, the investigators obtained a series of 11 images within 14 seconds, which apparently was enough to reveal the orientation of the genitals during intercourse.

The sequence of images started with a preliminary image of the woman lying on her back. She was then joined by her partner who began to make love to her in the man on top or missionary sex position, after which another image was taken.

The man then left the imaging equipment while the woman stimulated her clitoris to the preorgasmic stage, at which point a third image was taken.

The woman then recommenced stimulating her clitoris until she reached orgasm, after which another image was taken, and the final one 20 minutes after that.

The investigators used eight couples and obtained 13 sets of results, all of which involve clitoral orgasms rather than vaginal or G spot orgasms. There was no female ejaculation during the experiment, and several women said their orgasms were superficial - hardly surprising since it would be difficult to pleasure a woman inside an MRI scanner!

Six of the couples were only able to achieve partial penetration, although they were invited to repeat the procedure after the man had taken some Viagra to increase the hardness of his erection so that both penetration and intercourse could be investigated using the MRI scanner.

In the image which shows the anatomy of sexual intercourse, while a woman is lying on her back with the man in the male superior sex position, the root of his penis occupies almost one third of its length inside his body.

The other two thirds of its length are visible extending upwards at an angle of 120° to the root of the penis almost parallel to the woman's spine.

This bending of the penis occurred in all the couples who made love in the man on top position sex position and was unrelated to the depth of penetration.

Throughout intromission, pubic bones of the male and female participants were separated by about 4 cm (the woman’s being 4 cm higher than the man’s).

The uterus was elevated by 2.4 cm; since the participants described themselves as being only mildly sexually aroused, it's not clear whether this represents maximum or partial elevation of the uterus during intercourse.

Equally, several of the women described their orgasms as superficial, which is hardly surprising bearing in mind they were undergoing orgasm in the centre of a magnetic resonance imaging machine!

However, the investigators were able to establish that in the pre-orgasmic phase the interior of the vaginal wall increased in length by about 1 cm, and the uterus did indeed rise during intercourse.

This was a typical response during all the investigations. Interestingly enough, during sexual arousal when a woman was not engaged in intercourse, the position and size of the uterus did not change.

The investigators admit that they did not anticipate the difficulty that the men in this experiment would have maintaining their erection during the investigation.

By contrast, the women were able to achieve complete sexual response but with an orgasm which was clearly less intense than they would normally experience.

Only one couple were able to engage in intercourse without the help of Viagra.

So the question is: what was actually determined, was anything discovered about the anatomy of the genitals in this sex position that was not known before?

According to the investigators, “the caudal position of the male pelvis during intercourse, the potential size of the bulb of the corpus spongiosum, and the capacity of the penis in erection to make an angle of around 120° to the root of the penis, enabled penetration along the bottom of the symphysis up to the woman's promontorium or to the middle part of the sacrum almost parallel to her spine”.

What emerged very clearly was the during sex in the missionary position, the penis is not straight, but has the shape of boomerang.

When you take the root and the shaft of the penis together the average penis is in fact rather large (a total length of 22 cm).

The investigators observed that current scientific knowledge about changes in the female genitals during intercourse includes knowledge of (1) the way that the lining of the vagina changes during sexual activity, (2) engorgement of the vascular tissue of the urethra, and (3) movement of the uterus in an upward direction together with an effect called tenting in the vagina.

They make the observation that recent research has shown that the perineal urethra is embedded in the anterior wall of the vagina, surrounded on all sides by tissue that is clearly erectile except on the side where it relates to the wall of the vagina.

In other words, the bulbs of the vaginal vestibule are closely associated with the clitoral anatomy and urethra.

Since they observed no displacement of the uterus except when a woman was having sex, it may be inferred that the upward movement of the uterus during intercourse is caused by intromission of the penis.

They also make the observation that unlike Masters and Johnson they did not establish that the uterus increased in size during sexual arousal, an observation which may not be too surprising since it's unlikely that a uterus could increase in size by 5200% of the time described my Masters and Johnson, who in any event were establishing the size of the uterus through palpation.

Changes during sexual arousal

The MRI scans demonstrated very clearly that female sexual arousal produces changes in the anterior wall of the vagina.

These changes take place in the vaginal wall itself, and the MRI scans demonstrated that the anterior vaginal wall, with the urethra lying close behind, is indeed a sensitive area and may take part in the orgasmic reflex, although no direct evidence of this was obtained from the MRI scans in this study.

References

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12. Minh MH, Smadja A, De Sigalony JPH, Aetherr JF. Role du fascia de Halban dans la physiologie orgasmique feminime. Cahiers de Sexuol Clin 1981; 7: 169.
13. Hilleges M, Falconer C, Ekman-Ordeberg G, Johanson O. Innervation of the human vaginal mucosa as revealed by PGP 9.5 immunohistochemistry. Acta Anatomica 1995; 153: 119[Medline].
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16. Weijmar Schultz WCM, Van de Wiel HBM, Klatter JA, Sturm BE, Nauta J. Vaginal sensitivity to electric stimuli, theoretical and practical implications. Arch Sex Behav 1989; 18: 87-95[Medline].
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Tips for him to please her (2)
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Penis-Vagina Sex Imaging

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