The lower urinary tract in mammals consists of the bladder, a storage organ for urine, and the urethra through which urine is expelled to the outside. In the male the urethra extends outside the body as the penis. The lower urinary tract is innervated by pelvic (parasympathetic) nerves and hypogastric nerves which combine on each side at the pelvic plexus. Fibres pass to the bladder, urethra, genital organs, rectum and blood vessels. In addition to the pelvic and hypogastric nerves there is sacral outflow to skeletal muscle, such as the external sphincters of the urethra and anus, in the pudendal nerve and the sympathetic chain extends caudally with branches lying in the sacral outflow that supply the penis and other organs though not the bladder. Bladder evacuation and penile erection are produced by activity of the pelvic nerves whereas sympathetic activity usually dominates during urine storage and penile flaccidity, though it may not be essential for either.