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The muscles of the orbit that control the movement of the eye are known as extrinsic muscles or extraocular muscles because they arise externally and not from within the eye ball itself. There are six extraocular muscles, in total, that control eye movement. Of these six muscles, four known as the rectus muscles, control the movement of the eye in the cardinal directions up, down, left and right. The other two muscles, which are the oblique muscles, are responsible for counteracting the movement of the head by adjusting the movement of the eye accordingly.
The inferior rectus muscles is one of the four rectus muscles of the eye that are responsible for the movement of the eye in the cardinal directions. When we start with the eye being in a neutral position, the inferior rectus muscle is responsible for the depression, adduction and extorsion of the eyeball. So during adduction of the eye, this muscle is responsible not only for adduction but it also facilitates depression and extorsion movements. During abduction however, the inferior rectus is only responsible for depression.
The inferior rectus has its origin from the common tendon of the extraocular muscles known as the Annulus of Zinn. It has its insertion on the anterior inferior aspect of the eye and it receives motor innervation from the third cranial nerve, the oculomotor nerve (CN III).
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