The urinary bladder is a muscular sac for storing urine. The triangular base of the urinary bladder, the trigone, is defined by the two ureters that deliver the urine and the one urethra that drains the urine. When empty, the urinary bladder collapses and develops folds (called rugae) within the urinary bladder wall. As it fills, the folds become distended and the urinary bladder becomes spherical (smooth on the inside). The wall of the urinary bladder consists of three layers similar to those of the urethra: the mucosa, the muscularis (here called the detrusor muscle), and the adventitia. Circular smooth muscle fibers around the urethra form the internal urethral sphincter.