What are the eye muscles?
Six eye muscles move the human eye in all directions. There are four straight eye muscles and two oblique eye muscles.
Straight eye muscles
The four straight eye muscles are flat, thin muscles about one centimeter wide. They pull from the upper, lower, middle, and outer walls of the orbit (eye socket) to the corneal rim. The optic nerve runs in the space behind the eyeball that the eye muscles enclose in a pyramid shape.
The four straight eye muscles pull the eye in the following directions:
- upward and slightly inward (Musculus rectus superior)
- downward and slightly inward (Musculus rectus inferior)
- towards the middle – i.e. towards the nose (Musculus rectus medialis, the strongest of the eye muscles)
- outward (Musculus rectus lateralis)
Oblique eye muscles
- Pull outward and rotate downward inward (Musculus obliquus superior)
- Pull outward and rotate upward outward (Musculus obliquus inferior)
Ciliary muscle
Another eye muscle is the ciliary muscle, but it is not involved in the movement of the eye. Instead, the ciliary muscle function is to accommodate the eye:
The ciliary muscle is a part of the ciliary body (ray body) – the ring-shaped middle layer of the eyeball. Projections extend from the ciliary body to the lens of the eye, between which the lens suspensory ligament spans.
- When the ciliary muscle tenses, the suspensory ligament slackens and the lens curves more – following its own elasticity. This brings the near range into focus.
What is the function of the eye muscles?
The eye muscle function is to move the eyeball. A sharp image of our environment can only be produced in a small area of the retina, the point of central vision (fovea). At a distance of one meter, we can see sharply an area with a diameter of only nine centimeters.
Nevertheless, in order to perceive everything around us sharply, the eye must be able to scan with rapid movements every image that enters the eye from the outside. These gaze jumps are called saccades. In the process, the eye is repeatedly directed from the resting position to the next target at high speed. Thus, we do not grasp our entire field of vision at once, but “little by little”.
In contrast to the saccade, which is necessary for the detection of a stationary image, the perception of moving objects takes place by the eyes performing a following movement without jerking. This movement is much slower than the jerky saccades.
Both eyes must be moved absolutely synchronously to avoid double images. The eye must also compensate for head or body movements by moving in order to avoid blurring of the retina. The eye muscles make this possible.
What problems can the eye muscles cause?
Strabismus also occurs when an eye muscle is paralyzed. However, the squint angle then changes with the eye movement and is greatest when looking in the direction in which the main action of the paralyzed muscle lies. As a result, double vision occurs, which the affected person tries to compensate for by head posture.
Paralysis of the eye muscles can be caused by diseases of the eye socket (orbit) or by paralysis of the eye muscle nerves.
