The accessory organs of the digestive system are organs that are associated with digestion but not part of the digestive tract proper (this being the tract from mouth to anus, through which food travels and is digested). These organs include the salivary glands, pancreas, liver, and gallbladder. Each of these organs functions to aid digestion by increasing the speed and efficiency of breaking down food; for example, the salivary glands produce the enzyme amylase, which speeds up digestion of carbohydrates, and also allow food to more easily travel to the stomach. The pancreas produces an enzyme mix called "pancreas juice" which helps digestion in the small intestine, and also produces various necessary hormones in the body. The liver is one of the most important organs in the body; it produces a wide range of amino acids, enzymes, and bile, each of which helps to digest a certain type of food product (separating fats from carbohydrates, for example). The gallbladder aids the liver and concentrates its bile, making the digestion of fats faster and more efficient.
Except for the liver, each of these organs is "accessory," or unnecessary for pure digestive function. It is possible to live without the gallbladder, pancreas, and even salivary glands, but it is not possible to live without the liver. The liver's functions in the body are too important; without the liver, the body quickly accumulates a number of toxic substances normally neutralized by the liver.
Some texts list the teeth and tongue as accessory digestive organs, but since they are purely mechanical, they are not usually included.