Mouth and Pharynx

The mouth

The mouth (buccal cavity) is the first organ of digestion. The hard palate forms the front part of the roof of the mouth, and the soft palate forms the back part. The rest of the mouth is surrounded by muscle. The whole of the interior of the mouth is lined with a mucous membrane kept moist with saliva produced by salivary glands, which have ducts opening into the mouth. Saliva contains salts to buffer chemicals in the mouth, such as lactic acid, to prevent corrosion of tooth enamel; and salivary amylase, an enzyme that starts the digestion of simple carbohydrates. The term salivary glands refers to three pairs of glands called the parotid, sublingual and submaxillary glands, which between them produce 1–2 litres of saliva each day.

Food is reduced to a bolus for swallowing and is mixed with saliva by mastication. The bolus is pushed to the back of the mouth by the tongue. Contraction of the mylohyoid muscles squeezes the bolus into the pharynx and the tongue prevents it from moving back into the mouth.

At this time the soft palate is raised to prevent food from entering the nasal cavity.

The pharynx

The pharynx is a muscular chamber used in both respiration and digestion, and divides to form the larynx (voice box) and the oesophagus. Once the bolus touches sensory receptors at the back of the mouth, reflex swallowing occurs, breathing is inhibited, the larynx is raised to block the opening to the trachea, and the epiglottis is lowered to prevent food entering the respiratory system. As the larynx closes, the cricopharyngeal sphincter at the entrance to the oesophagus relaxes to allow entry of the bolus into the oesophagus.

The swallowing reflex

Swallowing, or deglutition, comprises a voluntary phase and an involuntary phase. The voluntary phase ends when the bolus is pushed by the tongue against the hard palate into the pharynx. Here, it touches the glossopharyngeal nerve, which triggers impulses in the ‘swallowing centre’ of the medulla oblongata in the brainstem, which responds immediately by sending a regulated sequence of impulses to the muscles of the pharynx, oesophagus, stomach and breathing muscles. Thus, the involuntary phase consists of the following three sub-phases:

  • Oral phase
  • Pharyngeal phase
  • Oesophageal phase

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