Since Portmann suggested surgical drainage of the endolymphatic sac in 1927, several similar procedures have been described. These procedures all share a common problem; the endolymphatic sac must be identified without injuring the brain and labyrinth. Anson and Donaldson(1965) have provided the otologist with a detailed and accurate description of anatomy of endolymphatic sac, but did not described a convient and reliable way of locating this structure through Trautmann's triangle via a mastoidectomy approach. Also it is difficult to find endolymphatic sac in spite of adequate exposure and high rate of false negative in tomogram finding was noted. The purpose of this study are to describe the location of endolymphatic sac within Trautmann's triangle using easily recognizable landmarks and the location of external aperture of vestibular aqueduct and endolymphatic sac in posterior cranial fossa using adult korean's twenty temporal bones. 

The result was as followed : 

1) The average distance between foveate impression and 

   (1) internal acoustic meatus --- 15.0(11.2-17.4)mm, 

   (2) cochlear aqueduct --- 14.2(7.9-17.4)mm, 

   (3) jugular fossa --- 9.4(5.0-13.1)mm, 

   (4) sigmoid sinus --- 6.3(3.0-15.1)mm, 

   (5) superior petrosal sinus --- 14.3(11.0-18.1)mm.

2) The average distance between external aperture of vestibular aqueduct and 

   (1) internal acoustic meatus --- 11.5(8.6-15.4)mm, 

   (2) cochlear aqueduct --- 13.7(10.0-17.1)mm, 

   (3) jugular fossa --- 10.2(6.4-14.2)mm, 

   (4) sigmoid sinus --- 10.6(5.4-16.5)mm, 

   (5) superior petrosal sinus --- 9.9(6.5-14.2)mm. 

3) The average diameter of foveate impression 

   (1) vertical diameter --- 10.7(7.4-15.6)mm, 

   (2) horizontal diameter --- 10.1(6.8-13.4)mm. The average length of external aperture of vestibular aqueduct --- 6.5(3.0-12.2)mm.

4) Shape of external aperture of vestibular aqueduct 

   (1) typical shape --- 14 cases, 

   (2) opposite shape --- 2 cases, 

   (3) horizontal shape---2 cases, 

   (4) pointed shape --- 2 cases. 

5) In surgeon's view, the average distance between endolymphatic sac and 

   (1) secondary genu of facial nerve --- 9.9(5.0-13.7)mm, 

   (2) Donaldson's line --- 6.8(3.6-9.6)mm, 

   (3) sigmoid sinus --- 11.6(6.0-19.4)mm, 

   (4) the inferior margin of PSSC --- 5.1(1.9-8.0)mm, 

   (5) the point of PSSC, bissecting by Donaldson's line --- 7.0(4.1-13.1)mm.