The trigonal muscle is of particular interest in a study of the mechanism of micturition, because of its peculiar anatomy and close relationship with the so-called vesical sphincters. The opening of the internal "sphincter" during micturition was formerly considered as an inhibitory action. We now believe that the contraction of the powerful trigonal muscle passing in the form of a bow through the weaker arcuate muscles at the vesical orifice helps mechanically to pull open the "sphincter." This view is confirmed by endoscopic and cystoscopic studies, as the trigonal muscle is seen to contract and pull open the vesical orifice. Obstruction to urination is generally followed by a partial or complete compensatory hypertrophy of the trigon. Upon removal of the obstruction, there is a corresponding compensatory trigonal atrophy. Occasionally following marked obstruction there is an undermining of the trigon, the hypertrophied muscle being dissected away from the bladder wall. Upon