EOMs. The lateral muscular branch supplies the lateral rectus, superior rectus, superior oblique, and levator palpebrae superioris muscles; the medial muscular branch, the larger of the 2, supplies the inferior rectus, medial rectus, and inferior oblique muscles.

The lateral rectus muscle is partially supplied by the lacrimal artery; the infraorbital artery partially supplies the inferior oblique and inferior rectus muscles. The muscular branches give rise to the anterior ciliary arteries accompanying the rectus muscles; each rectus muscle has 1–4 anterior ciliary arteries. These pass to the episclera of the globe and then supply blood to the anterior segment. The commonly held notion that the lateral rectus has fewer ciliary vessels than the other rectus muscles has been challenged by recent anatomical work, which shows that the number of ciliary vessels is similar for the lateral rectus and the other rectus muscles and that these vessels may, in fact, contribute substantially to the blood supply of the anterior segment.

Johnson MS, Christiansen SP, Rath PP, et al. Anterior ciliary circulation from the horizontal rectus muscles. Strabismus. 2009;17(1):45–48.

Venous System

The venous system parallels the arterial system, emptying into the superior and inferior orbital veins. Generally, 4 or more vortex veins are located posterior to the equator; these are often found near the nasal and temporal margins of the superior rectus and inferior rectus muscles. Although their number and positions can vary, the location of 2 vortex veins in the orbit is consistent: the inferotemporal quadrant just posterior to the inferior oblique muscle and the superotemporal quadrant just posterior to the superior oblique tendon.

Structure of the Extraocular Muscles

The important functional characteristics of muscle fibers are contraction speed and fatigue resistance. The eye muscles participate in motor acts that are among the fastest (saccadic eye movements) in the human body and among the most sustained (gaze fixation and vergence movements). Like skeletal muscle, EOM is voluntary striated muscle. However, EOM differs from typical skeletal muscle developmentally, biochemically, structurally, and functionally. The EOMs are innervated at a ratio of nerve fiber to muscle fiber up to 10 times that of skeletal muscle. This difference may allow for more accurate eye movements controlled by an array of systems ranging from the primitive vestibuloocular reflex to highly evolved vergence movements.

The EOMs exhibit a distinct 2-layer organization: an outer orbital layer, which acts only on connective tissue pulleys (see the section Pulley System), and an inner global layer, whose tendon inserts on the sclera to move the globe. The muscle fibers comprising the orbital and global layers can be either singly or multiply innervated.

Singly innervated fibers are fast-twitch generating and resistant to fatigue. Eighty percent of the fibers comprising the orbital layer muscle are singly innervated. Ninety percent of the fibers making up the global layer muscle are singly innervated, and they can be subdivided into 3 groups (red, intermediate, and white), based on mitochondrial content, with the red fibers being the most fatigue resistant and the white fibers, the least. The orbital singly innervated fibers are considered the major contributor to sustained EOM force in primary and deviated positions. Of all muscle fiber types, this type is the most affected by denervation from damage to the motor nerves or the end plates, as occurs after botulinum toxin injection.

The function of the multiply innervated fibers of the orbital and global layers is not clear. These fibers are not seen in the levator palpebrae superioris, and it is thought that they are involved in the