title: Relationship of the distal optic nerve sheath to the circle of Zinn.
abstract: BACKGROUND
  This study was designed to determine the relationship of the vascular circle of Zinn (CZ) and its branches to the termination of the subarachnoid space surrounding the optic nerve sheath, with particular reference to optic disc size.
  
  
  METHODS
  Serial sections of 29 normal human optic nerves were performed at 6-microm intervals. The position of the CZ and subarachnoid space were measured with WILD Heerbrugg objective graticules. The results were analysed with reference to the optic disc size.
  
  
  RESULTS
  The position of the CZ was variable but two major types were recognised. In type 1 the circle was located anterior to the distal sheath, and in type 2 posterior to the sheath. There were more small optic discs observed in type 2.
  
  
  CONCLUSIONS
  In this study the CZ was more posteriorly located in small optic discs. This location may induce risk factors contributing to the development of ischaemic optic neuropathy. The combination of small discs, posterior placement of the CZ, and anatomical variations in the vascular pattern may predispose to ischaemic events.
authors.affiliation: St. Thomas' Hospital, London, UK.
authors.forename: C D
authors.initials: CD
authors.lastname: Gauntt
authors.forename: T H
authors.initials: TH
authors.lastname: Williamson
authors.forename: M D
authors.initials: MD
authors.lastname: Sanders
mesh_terms: D002102:Cadaver; D006801:Humans; D009186:Myelin Sheath; D009898:Optic Disk / Q000098:blood supply / Q000166:cytology; D009900:Optic Nerve / Q000098:blood supply* / Q000166:cytology*; D012161:Retinal Artery / Q000033:anatomy & histology; D013346:Subarachnoid Space / Q000166:cytology