internal urethral orifice internal urethral orifice the internal urethral orifice is the opening of the urinary bladder into the urethra. it is placed at the apex of the trigonum vesicae, in the most dependent part of the bladder, and is usually somewhat crescent-shaped; the mucous membrane immediately behind it presents a slight elevation in males, the uvula vesicae, caused by the middle lobe of the prostate. internal urethral orificeurinary bladderthe interior of bladder.detailsidentifierslatinostium urethrae internumta98a08.3.01.028f a08.3.01.027mta23424fma85264anatomical terminology