Annulus of Zinn

The annulus of Zinn, also known as the annular tendon, circle of Zinn-Haller or common tendinous ring, is a ring of fibrous tissue surrounding the optic nerve at its entrance at the apex of the orbit. It is the origin for five of the six extraocular muscles.

Some older sources (such as Gray's Anatomy) distinguish between these terms more precisely, with the annulus tendineus communis being the parent structure, divided into two parts:
 * a lower, the ligament or tendon of Zinn, which gives origin to the Rectus inferior, part of the Rectus internus, and the lower head of origin of the Rectus lateralis.
 * an upper, which gives origin to the Rectus superior, the rest of the Rectus medialis, and the upper head of the Rectus lateralis. This upper band is sometimes termed the superior tendon of Lockwood.

It should not be confused with the Zonule of Zinn, though it is named after the same person (Johann Gottfried Zinn).