▁ca vern os um ▁u re th ra e ▁in ▁older ▁texts ▁and ▁in ▁the ▁adjacent ▁diagram ) ▁are ▁three ▁expand able ▁ere ct ile ▁t issues ▁along ▁the ▁length ▁of ▁the ▁pen is , ▁which ▁fill ▁with ▁blood ▁during ▁pen ile ▁ere ction . ▁The ▁two ▁corpor a ▁ca vern osa ▁lie ▁along ▁the ▁pen is ▁sh aft , ▁from ▁the ▁pub ic ▁b ones ▁to ▁the ▁head ▁of ▁the ▁pen is , ▁where ▁they ▁join . ▁These ▁form ations ▁are ▁made ▁of ▁a ▁sp on ge - like ▁t issue ▁containing ▁trab ec ula e , ▁irregular ▁blood - filled ▁spaces ▁l ined ▁by ▁end oth el ium ▁and ▁separated ▁by ▁connect ive ▁t issue ▁sept a . ▁ ▁The ▁male ▁an atom y ▁has ▁no ▁vest ib ular ▁bul bs , ▁but ▁instead ▁a ▁cor pus ▁sp ong ios um , ▁a ▁smaller ▁region ▁along ▁the ▁bottom ▁of ▁the ▁pen is , ▁which ▁contains ▁the ▁u re th ra ▁and ▁forms ▁the ▁gl ans ▁pen is . ▁ ▁Phys i ology ▁In ▁some ▁circumstances , ▁release ▁of ▁nit ric ▁ox ide ▁preced es ▁relax ation ▁of ▁mus cles ▁in ▁the ▁corpor a ▁ca vern osa ▁and ▁cor pus ▁sp ong ios um , ▁in ▁a ▁process ▁similar ▁to ▁female ▁ar ous al . ▁The ▁sp ong y ▁t issue ▁fills ▁with ▁blood , ▁from ▁ar ter ies ▁down ▁the ▁length ▁of ▁the ▁pen is . ▁A ▁little ▁blood ▁enters ▁the ▁cor pus ▁sp ong ios um ; ▁the ▁remainder ▁eng