The internal urethral orifice is placed at the apex of the trigonum vesicæ, in the most dependent part of the bladder, and is usually somewhat crescentic in form; the mucous membrane immediately behind it presents a slight elevation, the uvula vesicæ, caused by the middle lobe of the prostate.

When the urinary bladder fills, the orifice is closed and forms a flat sheet together with surrounding structure (Filling internal urethral orifice).

When the urinary bladderis emptying, the neck of bladder retracts like a funnel and the male urethra appears shortened (Emptying internal urethral orifice). 

This definition incorporates text from a public domain edition of Gray's Anatomy (20th U.S. edition of Gray's Anatomy of the Human Body, published in 1918 – from


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