▁ ▁Rel ations ▁The ▁tend on ▁of ▁this ▁mus cle ▁is ▁sometimes ▁separated ▁from ▁the ▁caps ule ▁of ▁the ▁shoulder - joint ▁by ▁a ▁b urs a , ▁which ▁may ▁communicate ▁with ▁the ▁joint ▁cav ity . ▁ ▁Inner v ation ▁The ▁su pr as cap ular ▁n erve ▁inner v ates ▁the ▁su pr asp in atus ▁and ▁in fr asp in atus ▁mus cles . ▁These ▁mus cles ▁function ▁to ▁ab duct ▁and ▁later ally ▁rotate ▁the ▁arm , ▁respectively . ▁ ▁Vari ation ▁The ▁in fr asp in atus ▁is ▁frequently ▁f used ▁with ▁the ▁ter es ▁minor . ▁ ▁Function ▁The ▁in fr asp in atus ▁is ▁the ▁main ▁external ▁rot ator ▁of ▁the ▁shoulder . ▁ ▁When ▁the ▁arm ▁is ▁fixed , ▁it ▁ad duct s ▁the ▁inferior ▁angle ▁of ▁the ▁sc ap ula . ▁Its ▁syn erg ists ▁are ▁ter es ▁minor ▁and ▁the ▁del to id . ▁ ▁The ▁in fr asp in atus ▁and ▁ter es ▁minor ▁rotate ▁the ▁head ▁of ▁the ▁hum er us ▁out ward ▁( external , ▁or ▁later al , ▁rotation ); ▁they ▁also ▁assist ▁in ▁carrying ▁the ▁arm ▁back ward . ▁ ▁Additionally , ▁the ▁in fr asp in atus ▁rein for ces ▁the ▁caps ule ▁of ▁the ▁shoulder ▁joint . ▁ ▁In ▁animals ▁From ▁an ▁evolution ary ▁pros pective , ▁the ▁p ector al ▁mus cles   – ▁the ▁p ector alis ▁major ▁and ▁p ector alis ▁minor   – ▁are ▁thought ▁to ▁have ▁evol ved ▁from ▁a ▁primitive