The little muscles that rotate our eyeballs with such enormous precision
Posted on homepage: 19 November 2007 (GMT+10)
The six small muscles that rotate an eyeball do so with enormous precision, directing its visual axis (line of sight) exactly toward the object of interest. But the complexity of the visual system is even more amazing, since we have two eyes and these need to rotate in unison.1
An extremely fine balance of pulls by all six of its rotatory muscles together determines the direction of gaze of the eyeball at any moment. None of the muscles works in isolation, but they act as a coordinated system in a most elegant way.
Four of the six muscles (figure 1), called the recti (Latin, straight), run straight forward from the back of the eye socket (the orbit) to the eyeball. These attach to the sclera (the white, fibrous outer tunic of the eyeball) in front of the eyeball’s centre of rotation.
The rectus muscles respectively rotate the eyeball principally in four directions—up, down, in (towards the nose) and out (towards the ear). Remarkably, even though these muscles tend to pull it backwards, the eyeball’s centre of rotation remains almost stationary. This is because a firm cushion of fatty tissue behind the eyeball resists their pull. Also, the rectus muscles are opposed by the two oblique muscles, the superior (upper) and inferior (lower), which tend to pull the eyeball forwards.
The oblique muscles are so named because they pull across the axis of the eye socket, namely forward and toward the nose. Unlike the rectus muscles, the oblique muscles attach to the sclera behindits centre of rotation. One of their actions is therefore to rotate the eye vertically—either down (the superior oblique) or up (the inferior oblique). These vertical actions are most efficient when the two eyes are converged (swung toward the nose), as we look at a close object.
Equally important, if not more so, is the function of the oblique muscles in rolling the eyeball about the visual axis. This occurs as a reflex reaction2—the ocular tilt reaction—in response to stimulation of the organ of balance in the inner ear (the vestibular apparatus). Thus, when we tilt our head to one side, our eyes rotate so we are able to keep an upright view of the surrounding world (figure 2).3
Designed for a purpose
Of all six eye muscles, the superior oblique4 is undoubtedly the most striking. From a mechanical engineering standpoint, it is obviously designed for a particular purpose. Let’s see why.
The feature that makes the superior oblique so distinctive is its pulley mechanism.5 Like the rectus muscles, the superior oblique arises at the back of the orbit and runs forward to the front. Instead of attaching to the eyeball directly, it passes through a tiny pulley (the trochlea: Latin, a wheel or pulley). This changes the direction of the superior oblique so that it pulls the eyeball forwards and inwards.
As the superior oblique passes forward along the upper inner angle of the orbit, it is rounded and muscular. Shortly before reaching the trochlea it becomes a thin, smooth and round tendinous cord. Then, as it passes through the pulley, it turns back at an acute angle so that its direction of pull is in line with the inferior oblique. The tendon terminates by fanning out to attach to the sclera along an arc.
The trochlea, through which the tendon of the superior oblique slides, is a small, snug sling of fibrous tissue, firmly attached to bone. It acts as a pulley, and two remarkable design features greatly reduce friction within it:
a) a small, saddle-shaped sliver of cartilage6 which forms a grooved bearing against which the tendon is bent through an acute angle; and
b) a tiny sac,7 secreting a lubricant, between the tendon and the cartilage.
Why do we have such an elaborate mechanical arrangement of the superior oblique rather than the simpler design of the inferior oblique? One reason, perhaps, is that space in the orbit above the eyeball is restricted by the presence of two other muscles, namely the superior rectus and the levator (which raises the upper eyelid).
The superior oblique tendon passes to its insertion under these two muscles and, because of its slenderness, does not interfere with their action. This long, thin tendon thus neatly overcomes the space problem, while the muscle’s thicker ‘belly’ is located toward the rear of the orbit where there is more room. By contrast, below the eyeball there are only two muscles to be accommodated, namely the inferior oblique and inferior rectus.
Clearly, unless all the different parts are present and operating properly, the entire system will not work. No-one in their right senses would suggest that a man-made device similar to the superior oblique and with such a specific function could have ever assembled itself by a long series of lucky accidents, no matter how long we waited. Yet this is what the notion of evolution requires us to believe. This is not to mention the utterly fantastic computerized control system orchestrating all twelve eye muscles, without which they would be useless.
Advocates of evolution argue that somehow life and DNA are different from man-made machinery. But to think like this is to attribute magical properties to life—the very idea they disavow absolutely. Surely, the beautiful design of the superior oblique tells us we have been created by the Supreme Engineer. He has given to mankind alone (unlike the animals) the ability to design machinery, to imitate His engineering feats and to appreciate good design when we see it.
References and notes
- The brain normally superimposes the two retinal images to perceive them as a single mental image (a process known as fusion). But to achieve this, the two visual axes have to be simultaneously directed toward the same object with an angular accuracy of about 1/10 of a degree; otherwise the subject will see two images, i.e. double vision. Yeshurun, Y. and Schwartz, E., Cortical hypercolumn size determines stereo fusion limits, Biol. Cybern. 80:117–129, 1999. Return to Text.
- A reflex is an involuntary response to a stimulus. Return to Text.
- Leigh, R.J. and Zee, D.S., The Neurology of Eye Movements, F.A. Davis Company, Philadelphia, pp. 3–4, 1983. Return to Text.
- Wybar, K., Ocular motility and strabismus; in: Duke-Elder S., (Ed.), System of Ophthalmology, Henry Kimpton, London, VI:423–425, 1976. Return to Text.
- About a decade ago it was discovered that the superior oblique is not alone in having a pulley mechanism. The rectus muscles, around the equator of the eyeball, are constrained by fibroelastic connective tissue sheaths which are anchored to the orbit and prevent side slippage when the eyeball rotates away from the straight-ahead position of gaze. Demer, J.L., Miller, J.M., Poukens, V. et al. Evidence for fibromuscular pulleys of the recti extraocular muscles, Investigative Ophthalmology and Visual Science 36:1125, 1995. Return to Text.
- Cartilage is the firm but flexible tissue (gristle) which for example covers joint surfaces bearing against each other. Return to Text.
- Such a sac is called a bursa. These closed sacs are lined with a specialized (synovial) membrane secreting a lubricant fluid. They are usually found or formed in areas subject to friction. Return to Text.