Organ (anatomy)

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Organ
Internal organs.png
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.[1] In anatomy, a viscus (/ˈvɪskəs/) is an internal organ, and viscera (/ˈvɪsərə/) is the plural form.[2][3]

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.

A hollow organ is a visceral organ that forms a hollow tube or pouch, such as the stomach, intestine, or bladder.

Organ systems[edit]

The liver and gallbladder of a sheep.

Two or more organs working together in the execution of a specific body function form an organ system, also called a biological system or body system. The functions of organ systems often share significant overlap. For instance, the nervous and endocrine system both operate via a shared organ, the hypothalamus. For this reason, the two systems are combined and studied as the neuroendocrine system. The same is true for the musculoskeletal system because of the relationship between the muscular and skeletal systems.

Mammals such as humans have a variety of organ systems. These specific systems are also widely studied in human anatomy.

The origin and evolution of organs[edit]

Relationship of major animal lineages with indication of how long ago these animals shared a common ancestor. On the left, important organs are shown, which allows us to determine how long ago these may have evolved.

The organ level of organisation in animals can be first detected in flatworms and the more derived phyla. The less-advanced taxa (like Placozoa, Sponges and Radiata) do not show consolidation of their tissues into organs.

Complex animals are composed of organs and many of these organs evolved a very long time ago. For example, the liver evolved in the stem vertebrates more than 500 million years ago, while the gut and brain are even more ancient, arising in the ancestor of vertebrates, insects, and worms more than 600 million years ago.

Given the ancient origin of most vertebrate organs, researchers have looked for model systems, where organs have evolved more recently, and ideally have evolved multiple times independently. An outstanding model for this kind of research is the placenta, which has evolved more than 100 times independently in vertebrates, has evolved relatively recently in some lineages, and exists in intermediate forms in extant taxa.[4] Studies on the evolution of the placenta have identified a variety of genetic and physiological processes that contribute to the origin and evolution of organs, these include the re-purposing of existing animal tissues, the acquiring of new functional properties to these tissues, and the novel interaction of distinct tissue types.[4]

Plants[edit]

The flower is the angiosperm's reproductive organ. This Hibiscus flower is hermaphroditic, and it contains stamen and pistils.
Strobilus of Equisetum telmateia.

The study of plant organs is referred to as plant morphology, rather than anatomy, as in animal systems. Organs of plants can be divided into vegetative and reproductive. Vegetative plant organs are roots, stems, and leaves. The reproductive organs are variable. In flowering plants, they are represented by the flower, seed and fruit. In conifers, the organ that bears the reproductive structures is called a cone. In other divisions (phyla) of plants, the reproductive organs are called strobili, in Lycopodiophyta, or simply gametophores in mosses.

The vegetative organs are essential for maintaining the life of a plant. While there can be 11 organ systems in animals, there are far fewer in plants, where some perform the vital functions, such as photosynthesis, while the reproductive organs are essential in reproduction. However, if there is asexual vegetative reproduction, the vegetative organs are those that create the new generation of plants (see clonal colony).

History[edit]

Human viscera

Etymology[edit]

The English word "organ" derives from the Latin organum, meaning "instrument", itself from the Greek word ὄργανον, órganon ("implement; musical instrument; organ of the body"). The Greek word is related to ἔργον, érgon ("work").[5] The viscera, when removed from a butchered animal, are known collectively as offal. Internal organs are also informally known as "guts" (which may also refer to the gastrointestinal tract), or more formally, "innards".

Aristotle used the word frequently in his philosophy, both to describe the organs of plants or animals (e.g. the roots of a tree, the heart or liver of an animal), and to describe more abstract "parts" of an interconnected whole (e.g. his philosophical works, taken as a whole, are referred to as the "organon").

The English word "organism" is a neologism coined in the 17th century, probably formed from the verb to organize. At first the word referred to an organization or social system. The meaning of a living animal or plant is first recorded in 1842.[5] Plant organs are made from tissue built up from different types of tissue. When there are three or more organs it is called an organ system.[citation needed]

The adjective visceral, also splanchnic, is used for anything pertaining to the internal organs. Historically, viscera of animals were examined by Roman pagan priests like the haruspices or the augurs in order to divine the future by their shape, dimensions or other factors. This practice remains an important ritual in some remote, tribal societies.[citation needed]

The term "visceral" is contrasted with the term "parietal", meaning "of or relating to the wall of a body part, organ or cavity".[citation needed] The two terms are often used in describing a membrane or piece of connective tissue, referring to the opposing sides.[citation needed]

Seven vital organs of antiquity[edit]

Some alchemists (e.g. Paracelsus) adopted the Hermetic Qabalah assignment between the seven vital organs and the seven classical planets as follows:[6]

Planet Organ
Sun Heart
Moon Brain
Mercury Lungs
Venus Kidneys
Mars Gall bladder
Jupiter Liver
Saturn Spleen

Modern definitions and ontology[edit]

The variations in natural language definitions of what constitutes an organ, their degree of precision, and the variations in how they map to ontologies and taxonomies in information science (for example, to count how many organs exist in a typical human body) are topics explored by writer Carl Engelking of Discover magazine in 2017 as he analyzed the science journalism coverage of the evolving scientific understanding of the mesentery.[7] He explored a challenge now faced by anatomists: as human understanding of ontology generally (that is, how things are defined, and how the relationship of one thing to another is defined) meets applied ontology and ontology engineering, unification of varying views is in higher demand.[7] However, such unification always faces epistemologic frontiers, as humans can only declare computer ontologies with certainty and finality to the extent that their own cognitive taxonomy (that is, science's understanding of the universe) is certain and final. For example, the fact that the tissues of the mesentery are continuous was something that was simply not known for sure until it was demonstrated with microscopy.[8] Because humans cannot predict all future scientific discoveries, they cannot build a unified ontology that is totally certain and will never again change. However, one of the points made by an anatomist interviewed by Engelking is that, finality aside, much more could be done even now to represent existing human knowledge more clearly for computing purposes.

See also[edit]

References[edit]

  1. ^ Widmaier EP; Raff H; Strang KT (2014). Vander's Human Physiology (12th ed.). ISBN 978-0-07-128366-3. [page needed]
  2. ^ "Viscus - Definition". Merriam-Webster Online Dictionary. Retrieved 14 December 2009. 
  3. ^ "Viscera". MeSH. Retrieved 14 December 2009. 
  4. ^ a b Griffith, Oliver W.; Wagner, G?nter P. (23 March 2017). "The placenta as a model for understanding the origin and evolution of vertebrate organs". Nature Ecology & Evolution. 1 (4): 0072. doi:10.1038/s41559-017-0072. 
  5. ^ a b Barnhart's Concise Dictionary of Etymology[page needed]
  6. ^ Philip Ball, The Devil's Doctor: Paracelsus and the World of Renaissance Magic and Science, ISBN 978-0-09-945787-9[page needed]
  7. ^ a b Engelking, Carl (2017-01-06), "We got the mesentery news all wrong", The Crux (a group blog by Discover writers). 
  8. ^ Coffey, J Calvin; O'Leary, D Peter (2016). "The mesentery: structure, function, and role in disease". The Lancet Gastroenterology & Hepatology. 1 (3): 238–247. doi:10.1016/S2468-1253(16)30026-7.