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. Prior to the adoption of sacrum, the bone was also called holy bone in English, paralleling German heiliges Bein or Heiligenbein (alongside Kreuzbein) and Dutch heiligbeen. 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). Others attribute the adjective ἱερόν to the ancient belief that this specific bone would be indestructible. 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. 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". The Latin word is found in the alternative Latin name of the sacrum, ossa clunium, as it were "bones of the buttocks". Due to the fact that the os sacrum is broad and thick at its upper end, the sacrum is alternatively called os latum, "broad bone". Other animals In dogs the sacrum is formed by three fused vertebrae. The sacrum in the horse is made up of five fused vertebrae. 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.