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Also in this week's column:

• What are the scientific reasons for having sex?
• What can you learn from the sound of someone's voice?
• What are the most widely practiced religions of the world?

Asked by Michel Durinx of Leiden, The Netherlands

The medical term for "crossed-eyes" is strabismus. Another medical term for it is heterotropia. But it goes by other popular names too including wandering eyes, squint eyes, and walled eyes. Walled eyes is often thought of as the opposite of crossed eyes.

Crossed eyes is when the eyes point inwards. Walled eyes is when the eyes point outwards. Strabismus is a chronic abnormality of the eye from the normal visual axes (ocular deviation). The the eyes don't point in the same direction.

Strabismos is Greek for "squinting". The type of strabismus is referred to as a tropia. Troupe is Greek for "turning". A prefix indicates the direction of the ocular deviation. So when one eye is rotated around its visual axis with respect to the other eye it is "cyclotropia". Cyclo is also Greek for "turning". When the ocular deviation is such that one eye turns inwards it is "esotropia" and when outwards it is "exotropia". When the ocular deviation is such that one eye turns upwards it is "hypertropia" and when downwards it is "hypotropia".

Eye muscles (extraocular muscles) bring the gaze of each eye to the same point in space. Strabismus occurs when there is a lack of coordination of these muscles. This results in poor binocular vision that affects depth perception.

Near or far-sightedness can result because the brain cannot fuse the two different visual images into one. In infancy, congenital strabismus can cause amblyopia. This is a condition in which the brain ignores input from the ocular deviating eye, although it is capable of normal sight.

Amblyopia is sometimes called "lazy eye". Sometimes when the infant's bridge of the nose is wide and flat and when there are skin folds in the corner of the eyes, there is an appearance of strabismus. This is called "false strabismus" or "pseudostrabismus". As the child grows, the skull develops, and the bridge of the nose narrows, the folds in the corner of the eyes go away. There is no problem with vision. The test for this involves the doctor shining a light into the child's eyes. The light's reflection off of the pupil should be in the same spot of each eye if there is no strabismus.

Stephen Juan, Ph.D. is an anthropologist at the University of Sydney. Email your Odd Body questions to s.juan@edfac.usyd.edu.au

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