The digestive system is the collective name used to describe the alimentary canal, some accessory organs and a variety of digestive processes which take place at different levels in the canal to prepare food eaten in the diet for absorption. The alimentary canal begins at the mouth, passes through the thorax, abdomen and pelvis and ends at the anus. It has a general structure which is modified at different levels to provide for the processes occurring at each level. The complex of digestive processes gradually breaks down the foods eaten until they are in a form suitable for absorption. For example, meat, even when cooked, is chemically too complex to be absorbed from the alimentary canal. It therefore goes through a series of changes which release its constituent nutrients: amino acids, mineral salts, fat and vitamins. Chemical substances or enzymes which effect these changes are secreted into the canal by specialised glands, some of which are in the walls of the canal and some outside the canal, but with ducts leading into it.
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