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In anatomy, an organ is a quantity of tissue that has formed into an organized collection of cells in order to cooperatively perform an overall function. Both plants and animals can have organs; the study of plant organs is referred to as plant morphology, while the study of animal organs is known as anatomy. A single organ is also known as a viscus (plural viscera).

Some organs, such as the liver, are solid masses of tissue. Other organs, such as the heart and the stomach, have one or more cavities inside. These cavities are typically used to transport material throughout the organism, as when the heart’s chambers circulate blood or the stomach receives food and transfers it to the intestines.


The word “organ” is derived from Latin and Ancient Greek, where it originally meant “instrument” in the sense of a thing that performs a function, not unlike a tool or even a musical instrument. While organs exist as independent entities within the organism, they often are part of larger organ systems that are composed of several interrelated yet distinct organs, each performing separate but complementary functions. For example, the digestive system is an organ system that includes the stomach, the small intestine, the large intestine, the liver, and the pancreas, among other organs. Each organ works with the other members of the system to accomplish the overall goal of acquiring nutrients for the body’s sustenance.


Anatomy, or the study of the body’s parts and their functions, has a long and colorful history. Anatomy can be thought of on two different scales: gross (or macroscopic) anatomy, which refers to the study of organs and organ systems with the unaided eye, and microscopic anatomy, which is the study of organs using microscopes and other means of ocular amplification.

In ancient Egypt, it was thought that organs either performed certain functions or pertained to specific deities, and Egyptians believed that organs needed to be preserved for use in the afterlife, just like the bodies they mummified. When an individual died, the body’s organs were stored in canopic jars within the burial chamber. Each organ was kept in a jar carved in the likeness of the god to whom it pertained. For example, the stomach was kept in a jar that resembled the god Duamutef, distinctive for his jackal-like head.

Throughout history, different cultures have developed associations between organs and specific personality traits. The most common example is the modern association of the heart with love, passion, and intense emotion. The association between love and the human heart is seen in such traditions as wearing a wedding ring on the ring finger of the left hand; it was once believed that the vena amoris (literally, “vein of love” in Latin) ran directly from the heart to this finger. All fingers on both hands, however, share the same type of vein structure. Similarly, the brain is often viewed as the seat of consciousness, and there is frequent philosophical disagreement about whether the soul resides in the brain, the heart, or somewhere else entirely. The eyes are sometimes considered the “windows to the soul,” and the digestive organs have even been linked to subconscious intuition, such as having a “gut feeling” about someone or something.

Many cultures, including the Greeks, Romans, Etruscans, Babylonians, Assyrians, and Hittites, practiced a form of augury, or fortune-telling, known as haruspicy. In this ritual, a haruspex would sacrifice an animal and then look at the condition, position, and other characteristics of its internal organs (usually the intestines) in order to foretell what would or would not happen in the future.

Long ago, the only way for scientists and physicians to study organs was to have access to a corpse, which was challenging due to religious and cultural taboos pertaining to death and decay. Only in the twentieth century did technologies such as the x-ray, magnetic resonance imaging (MRI), ultrasound, and positron emission tomography (PET) scanning allow physicians and scientists to noninvasively observe and study functioning organs in a live patient. It is even possible to observe what parts of the brain become more neurochemically active in response to stimuli such as flashing lights, which helps scientists understand what sections of the brain are responsible for particular types of information processing (long-term memory, short-term memory, sensory stimulation, emotion regulation, and so forth).

Science has also advanced make organ transplantation possible. A functioning and healthy organ is removed from one body and is surgically inserted (transplanted) into the body of another patient, whose original organ has either been damaged or suffered from a defect that prevented it from operating appropriately. It is possible to transplant entire organs as well as tissue such as skin or bone marrow. In a few cases it is possible for a living person to donate tissue or an organ—a healthy person has two kidneys, for example, but can live with only one—but most organ donors are either brain-dead or recently deceased.

A frequent problem with organ transplant procedures is organ rejection, in which the recipient’s body rejects the donated organ as an intrusion and treats it as an invasive disease, causing the immune system to attack the newly received organ or tissue. Doctors continue to study how organs operate so they can work to suppress this immune response and thereby increase the likelihood of success in organ-transplant procedures.


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