<P> In the study of anatomy, the term viscus is used to refer to an internal organ, and viscera is the plural form . </P> <P> Seventy - nine organs have been scientifically validated in the human body . </P> <P> In biology, tissue is a cellular organizational level between cells and a complete organs . A tissue is an ensemble of similar cells and their extracellular matrix from the same origin that together carry out a specific function . Organs are then formed by the functional grouping together of multiple tissues . </P> <P> The study of human and animal tissues is known as histology or, in connection with disease, histopathology . For plants, the discipline is called plant anatomy . The classical tools for studying tissues are the paraffin block in which tissue is embedded and then sectioned, the histological stain, and the optical microscope . In the last couple of decades, developments in electron microscopy, immunofluorescence, and the use of frozen tissue sections have enhanced the detail that can be observed in tissues . With these tools, the classical appearances of tissues can be examined in health and disease, enabling considerable refinement of medical diagnosis and prognosis . </P>

Difference between human organ system and animal organ system