How food is digested in the mouth

The boarder of the mouth is represented by the gum but in front of it, are mobile lips. Roofing the buccal cavity is the hard plate and behind it is the soft plate. In the floor of the mouth lies the large muscular tongue, which is attached to the back and free at the anterior end.

When solid food is taken into the buccal cavity it is mastication before swallowing. Mastication is a mechanical process that involves the crushing and chewing action of the teeth, reducing the solid food into finer, smaller particles that are well lubricated by the rolling action of the tongue. The three pairs of salivary glands namely, Parotid, Submaxillary and Sub-lingual each secrete saliva. About 1.5 dm3 of saliva is produced by humans each day. Saliva is a watery secretion and contains the following:

1. Water: Saliva is a watery secretion with over 99% of it being water.

2. Enzyme: Saliva contains the enzyme salivary amylase or ptyalin and lysozyme. The amylase hydrolyses starch to maltose. The lysozyme on the other hand helps to kill bacteria, which are potentially harmful, by catalysing the breakdown of their cell wall.

3. Mineral salts: Saliva also contains mineral salts e.g. sodium hydrogen carbonate and chloride ions. This helps to maintain a pH of around 6.5 - 7.5 which is the optimum for its action whrles the chloride ions that activates the enzyme salivary amylase.

4. Mucin: This is a sticky material which helps to bind food particles together, moistens and lubricates food and makes it easier to swallow.

Mastication provides a large surface area for the enzyme ptyalin to act. If starch is present in the food, it is hydrolysed into maltose. The food substance is rolled into a ball-like mass called bolus and then swallowed into the oesophagus during which the epiglottis closes the entrance into the trachea. The oesophagus is muscular in nature and consists of both circular and longitudinal muscles by whose contraction peristaltic motions are generated and waves are produced to move the food towards the cardiac sphincter. As the food reaches the cardiac sphincter, it relaxes and allows the food to enter into the stomach. Quickly the cardiac sphincter contracts to prevent regurgitation that is reverse movement of food.