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Anastomosis (plural anastomoses) refers to a form of network in which streams both branch out and reconnect. The term is used in medicine, biology, and geology.

Anastomosis is the surgical connection of two structures.^[1] It commonly refers to connections between blood vessels or connections between other tubular structures such as loops of intestine. In circulatory anastomosis, many arteries naturally anastomose with each other. For example: the inferior epigastric artery and superior epigastric artery. In surgery, surgical anastomosis occurs when a segment of intestine is resected and the two remaining ends are sewn or stapled together (anastomosed). The procedure is referred to as intestinal anastomosis.

Pathological anastomosis results from trauma or disease and may involve veins, arteries, or intestines. These are usually referred to as fistulas. In the cases of veins or arteries, traumatic fistulas usually occur between artery and vein. Traumatic intestinal fistulas usually occur between two loops of intestine (enetero-enteric fistula) or intestine and skin (enterocutaneous fistula). Portacaval anastomosis, by contrast, is an anastomosis between a vein of the portal circulation and a vein of the systemic circulation, which allows blood to bypass the liver in patients with portal hypertension, often resulting in hemorrhoids, esophageal varices, or caput medusae.

In evolution, anastomosis is a recombination of evolutionary lineage. Conventional accounts of evolutionary lineage present themselves as the simple branching out of species into novel forms. Under anastomosis, species might recombine after initial branching out, such as in the case of recent research which shows that humans and chimpanzees may have interbred after an initial branching out.^[2] A second case in which the idea of anastomosis finds application is in the theory of Symbiogenesis, in which new forms of life (species) are seen to emerge from the formation of novel symbiotic relationships.

In geology, anastomosis is used to refer to quartz (or other) veins displaying this property, which is often related to shearing in metamorphic regions. There is also a type of stream called anastomosing, which consists of a wide strath and multiple thalwegs divided by vegetated stabilized islands within subparallel banks.