At http://www.bmj.com/cgi/content/full/319/7225/1596
you can view MRI images of sexual intercourse performed in the
Netherlands along with the scientific commentary.

This is the (in)famous penis=boomerang study, but IMHO that's the
least interesting part of it.

For me the most interesting part of the paper was the following
discussion relating to the filling of the women's bladder during
intercourse:

"Changes during sexual arousal Magnetic resonance imaging showed
strikingly that during female sexual arousal changes occurred in the
anterior vaginal wall. These changes took place in the vaginal wall
itself (the engorgement as such is not visible on the images),
through the raising of the uterus, displacement of the uterus caused
by penetration of the penis, and through gradual filling of the
bladder. Histological studies 11 12 and immunohistochemistry13 have
shown that the anterior wall of the vagina has denser innervation
than the posterior wall. This is supported by clinical studies 14 15
and research into vaginal sensitivity to electric stimuli16 in which
the anterior vaginal wallwith the urethra behind itwas found to be
relatively sensitive. Hoch's concept of a clitoral-vaginal sensory
arm of the orgasmic reflex refers specifically to the anterior
vaginal wall and the deeper tissuesthe urinary bladder, the
periurethral tissues, and Halban's fascia15and our images support
this."

Best,

Zeno
(from another list)