Next time you are having your lunch, absentmindedly reading the paper while eating your sandwich, spare a thought for swallowing if you dare. When you get down to the mechanics of it, we chew and swallow for a reason, but why? Well, ingestion and swallowing (mastication) occurs in the mouth (buccal cavity) in order to:

• Mix ingested food with secretions from the salivary glands

• Form the bolus, the ball of mashed up food

• Make ingested food easier to swallow

• Increase the surface area available for enzymatic digestion

• Begin starch digestion through an enzyme present in saliva

• Permit food to enter the stomach and further structures of the gastrointestinal tract

• Ensure an adequate nutrition, as, with diseases which impair chewing and ingestion, food is prevented from entering subsequent sections of the gut, hampering physiological well being.

The Teeth

The teeth (32 in number) are set into sockets (or alveolus) in the maxilla in the upper jaw, and in the mandibular in the lower jaw. Teeth on the right tend to mirror those on the left side. The first frontal tooth is the incisor, then there is a lateral incisor, then a single canine, then the first and second premolar, then three molars. The exposed surface of the tooth is the enamel. Under the enamel is the pulp, this is vascular and contains nerves, blood vessels and lymphatics.

Chewing (Mastication)

The muscles involved in mastication are mainly innervated by the motor branch of the fifth cranial nerve, where nuclei in the brain stem are responsible for regulating mastication.

Swallowing (Deglutition)

Deglutition occurs when food - now called bolus - is sufficiently chewed. In the voluntary stage deglutition receptors in the pharynx are stimulated by the bolus causing conduction of impulses to the brain stem initiating the involuntary stages.

The mouth has several openings that must be closed during swallowing, in the pharyngal stage the soft palate is pulled upwards across the posterior naris, preventing food entering the nasal cavity. A sagittal slit, through which the bolus must pass is created by the pulling together of the pharyngeal folds either side of the pharynx. The vocal chords are then strongly approximated, pulling the larynx upwards and anteriorly, this causes the epiglottis to swing backwards over the larynx's opening, thereby preventing food from entering the lungs.

The upper 3 - 4cm of the oesophageal wall composes the pharyngeal-oesophageal sphincter, which relaxes, allowing food to pass into the oesophagus. Finally, the entire pharyngal muscle wall contracts, beginning a rapid peristaltic wave through the oesophagus.

Neural areas throughout the reticular substance of the medulla and lower portion of the pons varolii, collectively termed the deglutition centre, are responsible for controlling deglutition, and also specifically inhibit the respiratory centre halting respiration at any point to permit swallowing. After all, choking isn't much fun and it's rather nasty if food does enter the lungs, as all sorts of horrible infections can occur. Just as well swallowing is such a streamlined process, then.