The lateral rectus muscle is one of the six muscles which controls movement of the eye. These very small muscles are quite strong, and capable of very precise and controlled movements which allow the eye to focus quickly on a visible target. The extraocular muscles, as these muscles are known, are innervated by several different nerves which allow for a high degree of control in eye movements.
In the case of the lateral rectus muscle, the muscle is a straight muscle which runs laterally along the outside of the eye. It is responsible for abducting the eye away from the nose; when someone glances sideways to the right, for example, the lateral rectus muscle in the right eye is responsible for pulling the right eye over. The complementary muscle to the lateral rectus is the medial rectus, which pulls the eye in towards the nose. More properly in biological terms, the medial rectus could be considered the antagonist of the lateral rectus muscle.
This particular extraocular muscle is innervated by the sixth cranial nerve, also known as the abducens nerve, and is the only muscle innverated by this nerve. If the lateral rectus muscle becomes weakened, people will have trouble controlling their eye movements and may not be able to look to the side. The same problems can emerge when damage to the sixth cranial nerve has occurred. A neurologist can assess damage by having a patient move through a series of eye exercises to identify areas of weakness or hesitation.
Together with the other extraocular muscles, the lateral rectus muscle extends back from the side of the eye to meet up with the annulus of Zinn, a ring of tough tissue which surrounds the optic nerve. The connections with the annulus of Zinn force the muscles into a cone-like shape, narrow at the annulus and wide at the eye. The optic nerve passes through this cone to reach the eye.
Apparent weakness in the lateral rectus can be caused by a number of different things. Nerve damage to the sixth cranial nerve may be caused by a degenerative condition which involves the nervous system, for example, and someone can also develop a degenerative condition which attacks muscle tissue, weakening muscles like the lateral rectus. If weakness does develop, some screening tests can be used to learn more about the cause and to identify potential treatments which could address the issue.