This is the 2007 version. Click here for the 2017 chapter 04 table of contents.

Structures of the Eye

The frontmost layer of the eye is the cornea. It is a tough, clear covering, bathed in tears, very painful when scratched. We blink in order to keep the cornea moist. The cornea refracts light, so it is the first stage in the process of focusing the visual image.

The transparent liquid behind the cornea, called the aqueous humor, also refracts light and helps bring the image of the outside world closer to a focus. Behind the chamber containing the aqueous humor is the iris, the colored part of the eye. The color of the iris and thus the eyes is determined by the amount of pigment contained in them. Albinos, who lack pigment, have pink irises.


Structures of the eye

04E

What is the cornea? The iris? What is the pupillary reflex, and what is its function?

The iris has a complex internal structure, including a set of muscles all its own. The muscles allow the iris to change the size of the hole in the middle of the iris, called the pupil. Pupil size changes to compensate for changing levels of light: this is the pupillary reflex. If you are in a brightly lit room, your pupil will shrink or constrict ; if you are in a dimly lit room, your pupils will enlarge or dilate. The fully dilated pupil has 17 times greater area than the fully constricted pupil.

Why are fixed and dilated pupils a bad sign, in the Emergency Room?

Muscles in the iris are controlled by the brain stem, so the pupillary reflex works only as long as the brain stem is alive. Doctors check the pupillary response of unconscious people brought into an Emergency Room. If the pupil is fixed and dilated, wide open and unresponsive to light, the patient is dead.


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