15.7: Muscles of the Eye
The muscles of the eye are sophisticated structures that control eye movement and focus, allowing for the precise and rapid adjustments necessary for vision. The human eye is controlled by ten muscles — six extraocular muscles, three intraocular muscles, and one primary eyelid retractor muscle.
Extraocular Muscles
The six extraocular muscles surround the eyeball and control its movements. They are responsible for a wide range of eye motions, including looking up, down, left, right, and rotating the eye. These include the lateral rectus, which moves the eye outward, and the medial rectus, which brings it inward toward the nose. The superior rectus and inferior rectus elevate and depress the eye, respectively, while the superior oblique and inferior oblique muscles rotate the eye. The four recti — superior, inferior, lateral, and medial recti — start from a common tendinous ring that encircles the optic foramen and attaches to the sclera. However, the superior oblique muscle starts above the optic foramen, while the inferior oblique starts on the maxilla below the orbit, and both muscles connect laterally to the sclera. These muscles work in synchronized pairs, allowing for coordinated focus and enabling critical functions like depth perception.
Intraocular Muscles
The intraocular muscles are located within the eye and are responsible for internal adjustments. The ciliary muscle plays a vital role in focusing vision by controlling the shape of the lens; it contracts to thicken the lens for nearby objects and relaxes for distance viewing. The iris contains two muscles: the sphincter muscle, which constricts the pupil in bright light to reduce light entry and protect the retina, and the dilator muscle, which widens the pupil under low light conditions to allow more light to enter, enhancing vision.
Primary Eyelid Retractor
Apart from these, there is a muscle called the levator palpebrae superioris, which originates in the wing of the sphenoid bone and attaches to the upper eyelid. This muscle is responsible for opening the eye. It works against the facial muscle, orbicularis oculi, responsible for closing or protruding the eyelid.
