trigone of bladder a triangular region of the wall of the urinary bladder, an area in which the muscle fibers are closely adherent to the mucosa; its three angles correspond with the orifices of the ureters and urethra.Called also vesical trigone.a triangular region of the wall of the urinary bladder, the three angles corresponding with the orifices of the ureters and urethra; it is an area in which the muscle fibers are closely adherent to the mucosa.lfactory trigone the triangular area of gray matter between the roots of the olfactory tract. vesical trigone the smooth triangular portion of the mucosa at the base of the bladder, bounded behind by the interureteric fold, ending in front in the uvula of the bladder.