The mouth and digestive organs

Digestion means crushing food mechanically, splitting it with enzymes into small molecules and passing these substances into blood or lymph. The food route begins in the mouth and runs through the pharynx and oesophagus to the stomach, then to the small and large intestine and finally the rectum; the liver, pancreas and salivary glands assist. In the mouth teeth crush food, the tongue mixes it and tastes, and three pairs of large salivary glands (sublingual, submandibular, parotid) produce saliva; an enzyme in saliva starts splitting starch and lysozyme reduces microbes. There are 32 permanent teeth: 8 incisors, 4 canines, 8 premolars and 12 molars; there are 20 milk teeth, which start to be replaced by permanent ones at about 6–7 years. The surface of the crown is enamel, the hardest substance, with dentin and a pulp with vessels and nerves inside. Tooth care: brushing twice a day (morning and evening), eating sweets in moderation, not cracking hard things (nuts, hard sweets) with teeth, and visiting a dentist at least once a year. Food moves through the oesophagus by wave-like muscle contraction (peristalsis) into the stomach.