Radiological-histological studies showed that the normal mucosal units of the colon are bounded by the innominate lines. The lines are superficial identations of the mucosa with depths of 0.23 mm. No histological difference was demonstrated according to the three radiographic types found. These lines are demonstrable in about 90% of double-contrast examinations using 60-70% w/v barium. The colon should be prepared with dietary restriction, magnesium citrate, and contact laxative under sufficient hydration. The lines can also be demonstrated at endoscopy with dye scattering.