This is a cross section of the colon - the view you'd see if someone were to cut your colon in half.
Illustration by Donna Myers © 2007
The second half of the muscularis externa is comprised of a band of longitudinal muscle. It's referred to as "longitudinal" because it runs lengthwise along the colon. When this muscle contracts, it works in conjunction with the circular muscle to create a wavelike motion (called peristalsis) that moves waste material through the colon. For a familiar example, think of someone playing an accordian. When the longitudinal muscle contracts, it makes the colon shorter.
