An organ that is located within the body cavity (or in its extension, in the scrotum); it consists of organ parts that are embryologically derived from endoderm, splanchnic mesoderm or intermediate mesoderm; together with other organs, the viscus constitutes the respiratory, gastrointestinal, urinary, reproductive and immune systems, or is the central organ of the cardiovascular system. Examples: heart, lung, esophagus, kidney, ovary, spleen.