Inferior rectus muscle
Citation, DOI & article data
- innervation: inferior branch of the oculomotor nerve (CN III)
- origin: annulus of Zinn (tendinous ring)
- insertion: globe (anterior, inferior surface)
- primary function: one of two ocular depressors
- secondary function: one of the two ocular external rotators
- tertiary function: one of the three ocular adductors
Inferior rectus, along with the other rectus muscles, arises from the annulus of Zinn, the common tendinous ring at the apex of the orbit that surrounds the optic canal 1.
Inferior rectus is crossed by the inferior oblique muscle, which runs inferior to it as it crosses the floor of the orbit.
The primary action of the inferior rectus is to depress the eye (see figure 1) 1. However, because the apex of the orbit is placed medially in the skull, the orbital axis that the inferior rectus runs in does not correspond with the optical axis of the eye in its neutral position. This means that the inferior rectus has secondary actions of adduction and external rotation (see figures 2 and 3).
If the eye is abducted by the lateral rectus such that the optical axis lines up with the orbital axis, the inferior rectus produces ocular depression only, and is solely responsible for this movement. Thus, when the physician testing eye movements first asks the patient to follow their finger laterally then inferiorly in the familiar H-shape, the inferior rectus muscle (and the oculomotor nerve that supplies it) are being directly tested.
If the eye is adducted by the medial rectus, the orbital axis runs almost perpendicular to the optical axis, so the inferior rectus no longer produces effective ocular depression, and instead produces external rotation and adduction.
Rectus comes from the Latin rectos, meaning straight 1.
- 1. Moore KL, Agur AMR, Dalley AF. Clinically Oriented Anatomy. (2013) ISBN: 9781451119459
- 2. Netter FH. Atlas of Human Anatomy. (2018) ISBN: 9780323393225
- 3. Imaging of the Head and Neck. Thieme. (2012) ISBN:3131505311. Read it at Google Books - Find it at Amazon
- 4. Gray's basic anatomy. Churchill Livingstone. ISBN:1455710784. Read it at Google Books - Find it at Amazon