ors al ▁side ▁( the ▁area ▁facing ▁up wards ▁while ▁standing ). ▁These ▁surfaces ▁are ▁rough ▁for ▁the ▁attachment ▁of ▁lig aments . ▁The ▁b one ▁is ▁cur ved ▁longitud in ally , ▁so ▁as ▁to ▁be ▁con ca ve ▁below , ▁slightly ▁convex ▁above . ▁ ▁The ▁base ▁or ▁posterior ▁extrem ity ▁is ▁w edge - sh aped . ▁The ▁third ▁met at ars al ▁b one ▁art icul ates ▁proxim ally , ▁by ▁means ▁of ▁a ▁tri angular ▁smooth ▁surface , ▁with ▁the ▁third ▁c une iform ; ▁med ial ly , ▁by ▁two ▁fac ets , ▁with ▁the ▁second ▁met at ars al ; ▁and ▁later ally , ▁by ▁a ▁single ▁fac et , ▁with ▁the ▁fourth ▁met at ars al . ▁This ▁last ▁fac et ▁is ▁situated ▁at ▁the ▁d ors al ▁angle ▁of ▁the ▁base . ▁ ▁The ▁head ▁or ▁anterior ▁extrem ity ▁art icul ates ▁with ▁the ▁third ▁proxim al ▁ph al an x . ▁ ▁Mus cle ▁attach ments ▁▁ ▁The ▁second ▁and ▁third ▁d ors al ▁inter os se i ▁mus cles ▁attach es ▁to ▁the ▁third ▁met at ars al ▁b one . ▁The ▁second ▁d ors al ▁inter os se i ▁from ▁the ▁med ial ▁side ▁of ▁the ▁b one ▁and ▁the ▁third ▁d ors al ▁inter os se i ▁from ▁the ▁later al ▁side . ▁The ▁function ▁of ▁the ▁mus cle ▁is ▁to ▁spread ▁the ▁to es . ▁ ▁The ▁first ▁Plant ar ▁inter os se i ▁mus cle ▁origin ates ▁from