The Mouth –
Teeth firstly perform mechanical operation of disgestion. The incisor teeth, which are in front of the mouth, bites the food first. Canine teeth (the sharp one, next to front teeth) shreds food into the smaller parts as it is passed back to the bicuspids that continues tearing it into smaller particles. At end the molar teeth those are present at the back and last of the teeth row, finish the grinding and crushing the food.
The purpose of chewing by the teeth is it increases the surface area of the food so that it may be more easily penetrated by the digestive enzymes so, chewing the food is a very important part of digestion. It not only breaks the food down into more easily digestible particles, but also stimulates nervous impulses that cause the secretion of gastric juices. In that way it prepares the digestive system for the food to be swallowed.
It is known fact that front teeth, which tear and shred food, can exert a force of up to 80 pounds. While the grinding molars can put 100 to 250 pounds of force against food particles!!!
On the other hand tongue performs the other aspect of mechanical digestion in the mouth by moving the food back and forth, from side to side and mixing it with the saliva which is the continues secretion by three pairs of salivary glands.
The saliva prepares the food for swallowing by lubricating it with mucin, which gives saliva its slippery characteristic. There is important enzyme called ptyalin or amylase that is helpful to digest the starches in the food. It converts the starch to a sugar called maltose.
Ptyalin is the major factor in starch digestion, so all starchy foods should be chewed thoroughly and mixed well with saliva. Human beings, however, are not well adapted to eating starches, so the amount of starches in the diet should be restricted.
Saliva also has a solvent action upon food. It is only after the food is somewhat dissolved that it can be tasted.
In addition to ptyalin, saliva has an enzyme called lysozyme that digests bacterial cell walls, thus killing certain microorganisms. Saliva also has a cleansing action as its constant flow helps to dissolve and remove food particles from the teeth.
After mechanical and chemical digestion has progressed to a certain point in the mouth, the tongue gathers the food together into a small ball and then elevates the mass of food back into the pharynx of the throat. This is the first stage of swallowing and the beginning of the food's journey down to the stomach.
Mouth to Stomach
After food rolls off the tongue, it is no longer under voluntary control. It is now moved through the system under the control of the involuntary nervous system. Apart of self-induced vomiting, it is now up to the the body to move the food.
The food travels by the food pipe known as esophagus from mouth to the stomach. During the whole process, the food is passed further by one particular motion and is term as peristaltic (wave-like) motion. Now this is so strong that even a lying down person eats something, food would be going further and further for digestion process.
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