The lower intestinal tract, simply known as the intestines, bowels or gut, was a two-part anatomical system, made up of the small intestines and large intestines, designed to transfer digested food to the circulatory system or to expel waste, in the form of feces and urine, from the body. It was part of the digestive system and made up the second half of the digestive tract.
A diagram of the Human torso depicting the location of the large intestine was stored in the USS Enterprise's library computer database. This database was accessed by the Talosians in 2254. (TOS: "The Cage")
Though Joe Tormolen's 2266 intestinal wound was not severe and Doctor Leonard McCoy intervened in what should have been enough time, the lieutenant junior grade died anyway, due to giving up. (TOS: "The Naked Time")
The Human small intestine was lined with millions of villi to enable them to absorb nutrients. McCoy mentioned this in 2269, comparing a part of the cosmic cloud to a small intestine. (TAS: "One of Our Planets Is Missing")