English sacrum was introduced as a technical term in anatomy

English sacrum was introduced as a technical term in anatomy in the mid-18th century, as a shortening of the Late Latin name os sacrum "sacred bone", itself a translation of Greek ἱερόν ὀστέον, the term found in the writings of Galen.[15][15][16][10][17][18][19] Prior to the adoption of sacrum, the bone was also called holy bone in English,[20] paralleling German heiliges Bein or Heiligenbein (alongside Kreuzbein[21]) and Dutch heiligbeen.[20][22][23] The origin of Galen's term is unclear. Supposedly the sacrum was the part of an animal offered in sacrifice (since the sacrum is the seat of the organs of procreation).[24] Others attribute the adjective ἱερόν to the ancient belief that this specific bone would be indestructible.[22] As the Greek adjective ἱερός may also mean "strong", it has also been suggested that os sacrum is a mistranslation of a term intended to mean "the strong bone". This is supported by the alternative Greek name μέγας σπόνδυλος by the Greeks, translating to "large vertebra", translated into Latin as vertebra magna.[15][25] In Classical Greek the bone was known as κλόνις (Latinized clonis); this term is cognate to Latin clunis "buttock", Sanskrit śróṇis "haunch" and Lithuanian šlaunis "hip, thigh".[26][27] The Latin word is found in the alternative Latin name of the sacrum, ossa clunium, as it were "bones of the buttocks".[20] Due to the fact that the os sacrum is broad and thick at its upper end,[22] the sacrum is alternatively called os latum, "broad bone".[20][25] Other animals In dogs the sacrum is formed by three fused vertebrae. The sacrum in the horse is made up of five fused vertebrae.[28] In birds the sacral vertebrae are fused with the lumbar and some caudal and thoracic vertebrae to form a single structure called the synsacrum. In the frog the ilium is elongated and forms a mobile joint with the sacrum that acts as an additional limb to give more power to its leaps.