The internal urethral orifice is the opening of the urinary bladder into the urethra.
Anatomy It is usually somewhat crescent-shaped.
Relations It is formed by the neck of the urinary bladder. It opens at the apex/inferior angle of the trigone of the bladder, some 2-3 cm anteromedial to either ureteral orifice.
The mucous membrane immediately posterior to it presents a slight elevation in males - the uvula vesicae - caused by the middle lobe of the prostate.
See also Internal sphincter muscle of urethra
References
External links - "The Male Pelvis: The Urethra"