x.3.3 Coitus revisited

New insights in the sexual respons and sexual anatomy, but also new research instruments like the MRI, have also led to new ideas about the nature of coitus. Current knowledge of genital changes during sexual arousal in women relates mainly to the vagina (thickening of the vaginal wall due to vasodilatation, lubrication, widening of the vaginal cavity), the urethra (possible congestion of the vascular tissue of the urethra), and the uterus (elevation of the uterus = ‘tent effect’ + change in position of uterus + change in size of uterus). MRI research has shown that the position of the uterus hardly changes during sexual arousal, unless penetration by the penis causes it to do so. Neither is a change in the size of the uterus observed during sexual arousal. Notable changes are the rapid filling of the bladder during sexual arousal and the lengthening of the anterior vaginal wall, in particular, during arousal with or without coitus. 
Lengthening is not observed when the uterus is in retroversion-flexion (RVF). The ‘tent effect’ is also absent in this case. It would appear that a uterus in RVF overstretches the anterior vaginal wall to such an extent that changes as a result of sexual arousal either do not occur or no longer result in a visible lengthening or elevation of the uterus. The lengthening/elevation of the uterus in RVF does occur again during penetration.
During intercourse in the missionary position, the caudal position of the male pelvis in relation to the female pelvis, 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 enable penetration almost parallel to the spine. The penis has the shape of a boomerang. The MRI imaging gives a convincing impression of the size of the average penis in erection (root plus pendulous part is 22 cm) and of the space available in the female body. The angle between the pendulous part of the penis in erection and the root of the penis is outside the lesser pelvis. The root of the penis therefore contributes to the length of the penis in erection.