The inferolateral surfaces of the bladder are cradled between the levator ani muscles of the pelvic diaphragm and the adjacent obturator internus muscles above the attachment of the pelvic diaphragm. The superior surface is slightly domed when the bladder is empty; it balloons upward as the bladder fills.
Neck of bladder
The neck of the bladder surrounds the origin of the urethra at the point where the two inferolateral surfaces and the base intersect.