An accessory organ has indirect involvement in a bodily process. It thus aids a system's primary organs in conducting functions like digestion. Accessory organs are found in various areas of the body, including the skin and the abdominal and chest cavities. Some definitions also consider organs near other organs that serve no apparent purpose — such as the appendix — to be accessory organs.

Several body parts may be classified as accessory organs to the digestive system. For example, the teeth and tongue assist in chewing and preparing food for digestion. In turn, salivary glands provide the mouth with liquid saliva that helps break down food so it can be more easily swallowed and digested. The substance in the saliva that aids in this breakdown is called amylase. Saliva also makes food into a pasty substance that can move down the throat more easily.

The pancreas is another digestive accessory organ that produces amylase and sends it to the stomach. In addition, it produces other substances that help break down fats and proteins. The pancreas is considered an accessory organ because its more primary function lies in the endocrine system.

The liver conducts many activities in the body, including functioning as an accessory organ for the digestive system. Like the pancreas, the liver aids in sugar, fat, and protein absorption. The liver also serves as a storage warehouse for essential nutrients like iron and various vitamins. An organ near the liver, the gallbladder, is impacted by digestive functions because it releases liver bile as fat circulates through the digestive system.

Accessory organs are not limited to the digestive system. Both the eyes and the ears contain several components that work in conjunction to produce sight and hearing. The eyelids and the ears' pinna are two such examples. Existing mainly to protect the main sight organs — the eyeballs — from harmful substances, the eyelids also keep the eye from becoming too dry. As for the pinna of the outer ear, this curved structure helps direct sound waves into the inner ear, where the sound is recognized by the eardrum and translated by the brain.

Likewise, the skin contains several accessory parts, including the following: hair follicles, sweat glands, and melanocytes. Hair follicles and sweat glands produce hair and sweat, respectively. These substances protect and cool the skin. Meanwhile, melanocyte cells contain pigments that give skin its color.

Both male and female human reproductive systems contain many accessory organs as well. For example, the vas deferens and ejaculatory ducts of men help transport and expel the sperm produced in the primary male sex organs, the testes. In females, examples of accessory organs are the labia, which lubricates the vagina, and the mammary glands, which provide milk for infants.

Accessory organs are also not limited to humans. One example comes from the aquatic animal kingdom of fish. Some certain fish types have adapted to breathing out of water. They have air chambers consisting of specialized skin and blood vessels attached to their primary breathing structures, the gills. As such, these chambers can be considered accessory organs.

Although less commonplace, the term accessory organ may occasionally be used in reference to appendage organs that scientists have deemed mostly irrelevant. These organs are believed to be relics of evolutionary processes, and thus no longer useful for humans. The appendix serves as one such example.