Internal organs

Organ
Many of the internal organs of the human body.
Identifiers
FMA	67498
Anatomical terminology []

In biology, an organ or viscus is a collection of tissues joined in a structural unit to serve a common function. In anatomy, a viscus (/ˈvɪskəs/) is an internal organ, and viscera (/ˈvɪsərə/) is the plural form.

Organs are composed of main tissue, parenchyma, and "sporadic" tissues, stroma. The main tissue is that which is unique for the specific organ, such as the myocardium, the main tissue of the heart, while sporadic tissues include the nerves, blood vessels, and connective tissues. The main tissues that make up an organ tend to have common embryologic origins, such as arising from the same germ layer. Functionally related organs often cooperate to form whole organ systems. Organs exist in all higher biological organisms, in particular they are not restricted to animals, but can also be identified in plants. In single-cell organisms like bacteria, the functional analogue of an organ is called organelle.