The presence of fats in the chyme slows the movement of food along the intestine. The ileum projects a little way into the caecum so that, inthe presence of a pressure gradient, material can pass readily from the small to the large intestine but cannot easily pass in the opposite direction. The remainder of the large intestine is called the colon which leads to the rectum. In the large intestine there is a complete layer of circular muscle but there are only strips of longitudinal muscle, called taeniae coli. A strong contraction of the circular muscle occurs over a long stretch of the colon and moves the contents towards the rectum.
Mass movements occur more frequently after a meal, a phenomenon called the gastro-colic reflex.
For most of the time the rectum is empty, even if there are faeces in the adjacent part of the colon. The internal anal sphincter, which is made of smooth muscle, and the external anal sphincter, which is made of skeletal muscle, are contracted. When mass movements force faeces into the rectum and distend it, afferent fibres are stimulated. The external anal sphincter is kept contracted voluntarily until a socially acceptable moment, when it is relaxed. At the same time the longitudinal muscle of the rectum and last part of the colon, as well as the levator ani, contract.