Atypical muscular slips between superior and inferior rectus muscles, lateral view

The atypical muscle (marked by black asterisk) originates at the common tendinous ring (origin marked by black arrowhead) and passes laterally to the optic nerve (II) and the ophthalmic artery (OA). (a) Dissection of the specimen harvested from the right orbit. The lateral rectus muscle has been removed to expose the accessory muscle. (b) Dissection of the specimen harvested from the left orbit. One of the sub-branches derived from the inferior branch of the oculomotor nerve (IB) innervates the inferior slip of the accessory muscle (this small branch has been marked by a white arrowhead).

IR, inferior rectus muscle; IO, inferior oblique muscle; NIO, nerve to inferior oblique; SR, superior rectus muscle; III, oculomotor nerve.

(Source: Haładaj R, Wysiadecki G, Polguj M, Topol M. Bilateral muscular slips between superior and inferior rectus muscles: case report with discussion on classification of accessory rectus muscles within the orbit. Surg Radiol Anat 2018;40[7]:855-62. Creative Commons Attribution 4.0 International [CC BY 4.0] license, creativecommons.org/licenses/by/4.0.)