When food is put in the mouth, it is tested for taste and temperature by the tongue. Solid food is bitten off by the front teeth (incisors), and then chewed by the back teeth or molars. Even before the food is tasted, and during chewing, saliva pours into the mouth from salivary glands near the lower jaw.
Saliva moistens food, and the enzymes it contains start digestion. By the time it is ready to be swallowed; the original mouthful has been transformed into a soft ball, called a bolus, and warmed or cooled to the right temperature.
Though quick, this stage is in fact quite complex. First the tongue pushes the bolus of food up against the roof of the mouth and into the muscle-lined cavity at the back of the mouth: the pharynx. Once food is in the pharynx, several activities take place within the space of a couple of seconds to prevent swallowing from interfering with breathing.
From the pharynx, the bolus now passes into the oesophagus, or gullet, the tube joining the mouth to the stomach. The bolus does not just fall down the oesophagus because of gravity but is pushed along by waves of muscle action called peristalsis.
Except during eating, the oesophagus is kept closed by a ring of muscles called the cardiac sphincter which prevents the highly acid contents of the stomach from being regurgitated into the oesophagus. As a bolus of food passes down the oesophagus, the sphincter relaxes to open the pathway into the stomach.
The stomach is a collapsible muscular bag designed to store food (so that it is not usually necessary to eat small meals all day long), to mix food with various digestive juices, then to release it slowly into the intestine.
Food is mixed as the stomach wall contracts and relaxes and is moved along by waves of peristalsis. By the time it has spent two to six hours being processed in the stomach, the partially digested food has been converted by various chemicals to a liquid called chyme.
The stomach exit is guarded by a muscle known as the pyloric sphincter. As the waves of peristalsis push chyme through the stomach, the sphincter lets out chyme in small amounts into the small intestine.
The small intestine is in fact the longest section of the alimentary canal, measuring 20ft (6m) and getting its name not from its length but its width – about 1 ½ in. (4cm). The largest part of the digestive process takes place in the small intestine, through the action of digestive juices made not only by the intestine itself, but by the liver (bile) and the pancreas (pancreatic secretions).
As waves of peristalsis move chime along the small intestine, it is given another thorough mixing. When more food enters the stomach, a valve at the end of the small intestine opens and chyme passes into the large intestine, a tube about 3ft (1.5m) long and an average 7.5 cm (3 in.) in diameter.
In the large intestine, water is absorbed into the blood from the liquid remains of digestion. By the time these remains reach the rectum, they are in the form of solid faeces. Rather than the continuous contractions of peristalsis, the progressively more solid remains are moved along by giant propulsions that take place only a few times a day.
Finally, faeces enter the anal canal, which is kept closed by the last sphincter in the alimentary canal. In infancy, the sphincter opens automatically when the anal canal is full. But as the nervous system matures, we learn to override the automatic signals.
The alimentary canal processes approximately 35 tons of food during the course of an average life of 70 years. No wonder, then, that it sometimes goes wrong. Some ills of the alimentary canal are so common they have become household words – ulcers, appendicitis, constipation, diarrhoea and heartburn, to name a few. While some are unavoidable, there are ways to keep the digestive system healthy.