Blood vessels. Coloured scanning electron micrograph (SEM) of a resin cast of blood vessels in a colon. The colon is the part of the large intestine extending from the secum to the rectum. Its primary role is in the extraction of water from faeces. The cast was made by injecting resin into the blood vessels, followed by chemical digestion of the surrounding tissue. The interior of the colon is composed of a net-like mucous membrane.