The examination of the entrails of sacrificial animals was, without doubt, one of the most widespread forms of provoked divination in ancient Mesopotamia, both in time and space. The first evidence dates from the end of the third millennium, and manuscripts referring to extispicy have been found all over Middle East, dating from all periods of cuneiform culture.
The liver was sometimes called the "Tablet of the Gods". Diviners first distinguished between the constituent parts of the organ and the marks left on it. The liver was then subdivided into sections and subsections that formed an artificial grid for the diviner to read: each part had a specific significance, dependent on its characteristics (colour, shape), its orientation, the presence or absence of a foreign body, etc. All sections and subsections were given technical names, which were often drawn from everyday language but whose meaning and reading were particular to extispicy.
A specialist diviner, called bārû in Akkadian (lit. "the seer"), was educated to read the signs of the liver. First he ritually posed a question to the gods of divination, Šamaš and Adad. Then he oversaw the sacrifice of the animal (mostly sheep). He then looked for a yes/no answer in the entrails of the animal, adding the positive and negative signs from each parts of the grid to get his final answer.
The compendium of sacrificial omens, generally called Bārûtu "the art of the seer", contained many thousands of omens relating to the readings and interpretations of the animals' entrails. The oldest date from the early second millennium, but most of surviving examples are from early first-millennium Assyria. This series, whose omens are structured with a protasis and an apodosis like all other divinatory series, consisted of ten chapters, distributed across approximately 100 Tablets. In all, the chapters successively covered the entire sheep, including its behaviour before the sacrifice (e.g., SpTU 1, 72 [/cams/gkab/P348493/]).
In the CAMS/GKAB corpus, all manuscripts of Bārûtu come either from Huzirina or from Seleucid Uruk, but they cannot all be precisely placed in the "standard" series. Several commentaries on the series have been found in Uruk, while a divinatory model of a sheep's lung is the only extispicy-related find from Kalhu.
Marie-Françoise Besnier
Marie-Françoise Besnier, 'Signs from entrails: Bārûtu', The Geography of Knowledge, The GKAB Project, 2019 [http://oracc.museum.upenn.edu/cams/gkab/theworldoftheipu/divinatoryseries/signsfromentrails/]