The excellence of latter is that its incision can be adjusted optional . 4 . 17 . 5 % of specimens have angled variation of the back wall of petrous bone 约有17 . 5颞骨岩部后骨板有成角变异( 7侧) ，且以后外缘为多。
2.
An anterior sigmoid sinus tend to be observed when the distance between petrous back wall and external auditory canal is shorter . 5 . 11 / 40 ( 27 . 5 % ) posterior lip of internal acoustic meatus cover facial nerve more than 1 / 3 气化的乳突其颞骨岩部后骨板与外耳道后壁之间距离较大，外耳道后壁和颞骨岩部后硬脑膜之间距离也反映了乙状窦前移程度，该距离愈小，乙状窦愈有前移倾向。
