It may seem easy to put food in your mouth, chew it and swallow it, but there is, as usual, more than meets the eye in this ‘simple’ action. The processes of digestion actually begin in the mouth.
The process of mastication, or chewing food, breaks it into smaller fragments by an up and down movement of the jaw, together with a side to side grinding action. Of the four muscles that control mastication, the most powerful are the musculus temporalis, which originates from a broad sheet over the side of the skull, and the chewing muscle (musculus masseter), which is the bulky muscle over the jaw, both of which act to close the jaws. Others, the pterygoid muscles, lie inside the lower jawbone, or mandible, and not only help the up and down action, but also the side to side movements. Between them, these muscles can exert a force of hundreds of pounds, by way of the molars, on food. When we chew our food, it is broken down into small particles, which because of their relatively large surface area, give digestive juices a head start and speed up the digestive process. Chewing also lessens the chance of damage to the lining surface of the bowel that might be caused by the passage of large fragments of food.
Saliva is a clear fluid produced in the mouth by the salivary glands. Although it consists of about 98 per cent water, it does contain a number of important constituents either dissolved in it or excreted with it. One of these is mucin which thickens the saliva, acts as a lubricant and is also mildly antibacterial. Also secreted is the digestive enzyme amylase. Small amounts of other enzymes are secreted, in addition to special antibodies which specifically react with certain disease germs. Saliva has numerous functions. It moistens dry food and dissolves substances such as salt or sugar. This is essential for the function of the taste buds. The mucin, in lubricating the food, not only helps both mastication and swallowing, but also aids lubrication of the mouth, which is essential for the production of speech. Saliva does little to digest food, but amylase, which breaks down starch before it is neutralized by the action of stomach acid, can continue its actions for up to twenty minutes stored in the middle of a bolus, the swallowed mass of food. Saliva also acts to minimize the effects of acids or alkalis in the mouth. In addition it has a cleansing action, washing over the teeth and flushing bacteria into the stomach.
There are three main groups of salivary glands, the parotid glands, situated at the angle of the jaw (these become swollen with mumps), the submandibular glands under the jaw, and the sublingual glands under the tongue. In addition, there are many salivary glands called buccal glands within the lining of the cheek. The parotid gland, the largest, secretes a fairly watery – serous – saliva through the parotid duct, which opens beside the second upper molar tooth. The submandibular gland secretes a mostly serous secretion together with some mucus, whereas the subligual gland secretes mostly mucus. The secretions of all these glands are controlled by the autonomic nervous system as a result of a number of reflexes. Saliva production is stimulated by the sight, smell or even the thought of food, but taste is a more powerful stimulant. Saliva secretion is increased during mastication and during swallowing to help wash the food bolus down the oesophagus.
The tongue is a powerful muscular structure essential for taste, speech and assisting chewing. Its main functions are to initiate swallowing and to control the exit of air from the mouth, making speech possible. The tongue is anchored to the floor of the mouth. Its muscles originate from the mandible, the hyoid bone (a small U-shaped bone in the front of the neck) and the base of the skull. These change the position of the tongue but there are also muscles within it that alter its shape. The upper surface, or dorsum,-of the tongue is thickened to cope with the continual contact with food.
In addition to the mechanical functions of the tongue, in moving food and in speech, it has important sensory qualities. Specialized nerve endings sense the temperature and texture of the food. Taste buds lie at the back of the tongue in structures known as the circumvallate papillae and are also found in the fungiform and foliate papillae over the body of the tongue. Taste buds pick up four different tastes: sweet, salt, sour and bitter. The latter is a useful defence against natural poisons. The other taste sensations are perceived by the sense of smell.
The cheek assists with mastication by helping to direct the bolus of food into the path of the molars. Like the rest of the digestive tract, it is lined by a mucus-secreting layer and contains buccal glands which secrete small quantities of saliva.
Functions of the throat
The pharynx – the passage through which swallowed food passes – is the part of the throat lying at the back of the mouth. It forms a muscular funnel which leads to the oesophagus and the larynx – the voicebox – and entry to the air passages. During swallowing, the tongue pushes food backwards and arches so that it forms a deep slope for it to enter the pharynx. During this process the vocal cords shut tight to close the larynx, and the epiglottis, a cartilaginous flap, acts first by being raised as a shield to direct food away from the larynx, and then by flipping over, acting as a temporary lid to stop small fragments entering. If any food or fluid enters the larynx (goes down ‘the wrong way”) a powerful coughing reflex is initiated, leading to its immediate expulsion.