More information about the Large Intestine

The large intestine is about 5 ft in length. It has an inner circular layer of muscle, and an outer longitudinal layer of muscle which consists of three strips called taenia coli. These are a little bit shorter than the bowel itself, so it is gathered into a series of saccules, or sack-like swellings. The end of the bowel - the anus - has two sphincters, the inner of which is involuntary, and the outer of which is under voluntary control.

The main function of the large bowel is the absorption of water and salts, and to act as a store for waste material. The continuous contractions of the small bowel are not transmitted to the large bowel (if they were we'd be up all night using the lavatory) - rather contractions are stimulated by the presence of food in the stomach. They die down overnight and reach a peak in the early morning, usually after breakfast. The contractions are under the control of two sets of nerves which run beneath the mucosa and within the circular muscle layer. The contractions have the effect of rolling the contents of the large bowel over, mixing them rather like in a cement mixer, so that it is all exposed to the absorptive surface of the intestine. In this way most of the water that enters the large bowel is absorbed, as is the gas that is produced by the action of the bacteria which occupy the bowel.

The contents of the bowel consist of undigested food material such as cellulose, water, bile pigments that give it its colour, and bacteria that breakdown some of the constituents producing gases such as hydrogen, methane and carbon dioxide. The bacteria are called intestinal flora, and they can be affected by antibiotics, which is why antibiotics often cause diarrhoea as a side effect.

Besides churning contractions there are also propulsive contractions which propel the contents towards the anus. Usually the rectum - the last five inches or so before the anus - is empty of faeces. The presence of faeces in the rectum stimulates the urge to defaecate, when there is a longitudinal contraction of the colon, and a relaxation of the sphincters. If it is socially inconvenient, voluntary control of the external sphincter prevents embarrassment and the reflex dies down. Repeated suppression of the normal reflex drive to defaecate may lead to chronic constipation.

During intra-uterine life the bowel is inactive, unless the foetus is subject to stress (when it may pass the contents of the bowel, called meconium, in to the amniotic fluid.) After birth, normal bowel activity develops and the bowel becomes colonised by bacteria which are essential for digestion. There is, of course, no control of bowel function in the infant, since all activities are reflex: the achievement of continence occurs when the toddler learns to override the reflex drive to empty the bowel until it is socially convenient - which requires maturity of the neurological control of the bowel (neurological means the nervous system). Most children become continent by the age of 2½ or 3; some sooner and some later. The more anxious and obsessive the parents, the more a problem it becomes. Best to let nature take its course, while keeping on with nappies (which are a great help).