In human anatomy, the ureters are tubes made of smooth muscle fibers that propel urine from the kidneys to the urinary bladder. In the adult, the ureters are usually 25–30 cm (10–12 in) long and ~3–4 mm in diameter. Histologically, the ureter contains transitional epithelium and an additional smooth muscle layer in the more distal one-third to assist with peristalsis. In humans, the ureters arise from the renal pelvis on the medial aspect of each kidney before descending towards the bladder on the front of the psoas major muscle. The ureters cross the pelvic brim near the bifurcation of the iliac arteries (which they cross anteriorly). This is a common site for the impaction of kidney stones (the others being the ureterovesical valve, where the ureter meets the bladder, and the pelvouteric junction, where the renal pelvis meets the ureter in the renal hilum). The ureters run posteroinferiorly on the lateral walls of the pelvis and then curve anteriormedially to enter the bladder through the back, at the vesicoureteric junction, running within the wall of the bladder for a few centimetres. The backflow of urine is prevented by valves known as ureterovesical valves. In females, the ureters pass through the mesometrium and under the uterine arteries on the way to the urinary bladder. An effective phrase for remembering this anatomical relationship is "water (ureters) under the bridge (uterine arteries or vas deferens)." A cross section of the ureter reveals three layers of tissue: 1) An inner mucosa consists of transitional epithelium covered by a lamina propria of connective tissue. Mucus secretions protect the ureter tissues from the urine. 2) A middle muscularis layer consists of longitudinal and circular layers of smooth muscle fibers. The muscle fibers force urine forward by peristalsis. 3) The outer adventitia consists of areolar connective tissue containing nerves, blood vessels, and lymphatic vessels. Ureters are also found in all other amniote species, although different ducts fulfill the same role in amphibians and fish.