As food enters the oral cavity, bite-size chunks of food are cut off by the upper
and lower incisors. These chunks are about the right size for the mouth to handle.
There are two key facts about digestion:
a. First, digestion is a chemical process. Food is broken down into its
constituent parts through the process of hydrolysis.
b. Second, this chemical process takes place only at wet surfaces of the food.
Food processes are broken down into smaller and smaller pieces through the
process of mastication, or chewing. This greatly increases the total surface area of the
food. The grinding and crushing are accomplished by the premolar and molar teeth.
Keeping the food between the surfaces of the grinding teeth are the tongue and the
The fluids secreted into the oral cavity by the salivary glands and the buccal
glands are collectively known as saliva. These fluids serve to wet the surface areas of
the food particles. Saliva also dissolves some of the molecules of food items. These
dissolved molecules are tested by the taste buds. Food particles are held together as a
bolus by the mucus, which also makes the bolus somewhat slippery. (para 6-7)
The bolus is moved posteriorly out of the mouth into the pharynx and then down
through the esophagus to the stomach. Both the upper and lower air passageway must
be protected as the bolus passes through the pharynx. (para 6-8)
The actions of the tongue are produced by its intrinsic muscles and the muscles
of the hyoid bone.
As the bolus approaches the pharynx, the upper air passageway is closed by the
soft palate, which also tenses to resist the pressure from the bolus. (para 6-9)
Wavelike contractions of the three pharyngeal constrictor muscles force the bolus
into the beginning of the esophagus.
As the larynx is raised along with the tongue and the hyoid bone, its epiglottis
turns down over the opening of the larynx. Thus, food is prevented from entering the
The esophagus is a tube with muscular walls. It extends from the pharynx
above, through the neck and thorax, to the stomach in the abdomen. Wavelike
contractions, called peristalsis, move the bolus through the esophagus to the stomach.