Eye muscles - 2

Extraocular muscles are the eye muscles outside the eyeball. There are seven of them - six attached to an eyeball and seventh to the upper eyelid. Blinking is a normal reflex due possible due to movement of eye muscle attached to upper eyelid. Blinking spreads drops of tear onto the cornea and keeps the cornea wet and transparent.

The six extraocular eye muscles move the eye outward, away from the nose or inward toward the nose. They can move the eye upward or downward. As a team, they can shift gaze from one object to another, or follow a moving object like a running deer.

The eye muscles can also orchestrate convergence or divergence of eyes and focussing eyes when peering at objects and adjusting to head movement

So far we have considered about eye muscles on the outer side of eye. But the eye has muscles internally too. These are the smooth muscles in iris and in ciliary body.

The iris is the circular ring like, adjustable diaphragm. It is a contractile part of choroid layer, the second layer of the eye. The iris covers the eye lens partly. It is in front of the eye lens and divides the part in front of lens into two chambers. It is about 12mm in diameter and has a circumference of 37 mm. In humans in its center is a circular aperture, the pupil. The pupil is not circular in all animals. The diameter of pupil may vary from 1 - 8 mm.

The muscles of the iris are in the form of smooth muscle bundles, which are in concentric circles. The muscles of the iris are sphincter pupillae and dilator pupillae.

These muscles are responsible for making the pupil wider or narrower. The change in size of pupil generally, is in response to light intensity.

Dilation of pupil occurs in darkness and when we are in fear or excited emotionally. Dilator pupillae muscles cause dilation.

In bright conditions the action of sphincter pupillae brings about the contraction of the pupil.

The ciliary body is between iris and choroid. It is a ring like thickening of tissue in choroid layer and is about 5 mm wide. It contains ciliary muscles. It helps in stabilizing eye lens and in accomodation of the eye. If you take a cross section of the ciliary body it is approximately triangular in shape.

Take a look at iris and the ciliary body http://www.vision.ca/eye/ciliary_iris.ht...

And http://www.stlukeseye.com/anatomy/Ciliar...

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