▁being ▁the ▁two ▁most ▁common ▁variations . ▁At ▁the ▁insert ion ▁the ▁tend on ▁of ▁the ▁ext ensor ▁dig itor um ▁bre vis ▁manus ▁often ▁joins ▁the ▁ext ensor ▁indic is ▁propri us , ▁although ▁it ▁also ▁occurs ▁when ▁the ▁ext ensor ▁indic is ▁propri us ▁is ▁absent . ▁It ▁was ▁also ▁reported ▁to ▁co exist ▁with ▁the ▁ext ensor ▁med ii ▁propri us , ▁another ▁an atom ical ▁variation ▁in ▁the ▁ext ensor ▁compart ment ▁of ▁the ▁hand . ▁The ▁mus cle ▁is ▁supplied ▁the ▁posterior ▁inter os se ous ▁n erve ▁and ▁posterior ▁branch ▁of ▁the ▁anterior ▁inter os se ous ▁ar tery . ▁ ▁Vari ation ▁ ▁The ▁ext ensor ▁dig itor um ▁bre vis ▁manus ▁was ▁found ▁to ▁be ▁present ▁in ▁ 4 % ▁of ▁the ▁population , ▁and ▁there ▁was ▁no ▁correlation ▁between ▁gender , ▁side ▁or ▁ancest ry . ▁it ▁inserted ▁to ▁the ▁index ▁finger ▁and ▁the ▁middle ▁finger ▁in ▁ 7 7 % ▁and ▁ 2 3 % ▁of ▁the ▁cases , ▁respectively . ▁It ▁occurred ▁bil ater ally ▁in ▁ 2 6 % ▁the ▁total ▁cases . ▁ ▁Development ▁ ▁At ▁some ▁point ▁during ▁the ▁emb ry onic ▁development , ▁the ▁prec ursor ▁ext ensor ▁mass ▁differenti ates ▁into ▁three ▁layers : ▁radial , ▁super f icial , ▁and ▁deep . ▁The ▁ext ensor ▁dig itor um ▁bre vis ▁manus ▁may ▁have ▁been ▁origin ated ▁from ▁the ▁deep ▁layer , ▁a ▁typical ▁location ▁where ▁most ▁of ▁the ▁variations ▁take ▁place . ▁Some ▁authors ▁believe ▁that