Page 88 - 84 human physiolofy part-1

Chapter 5

Anatomy of the Eye

Cross-section of the human eye.
The human eye is a elongated ball about 1-inch (2.5 cm) in diameter and is protected by a bony
socket in the skull. The eye has three layers or coats that make up the exterior wall of the eyeball,
which are the sclera, choroid, and retina.

Sclera
The outer layer of the eye is the sclera, which
is a tough white fibrous layer that maintains,
protects and supports the shape of the eye.
The front of the sclera is transparent and is
called the cornea. The cornea refracts light
rays and acts like the outer window of the
eye.

Choroid
The middle thin layer of the eye is the
choroid, also known as the choroidea or choroid coat, it is the vascular layer of the eye lying
between the retina and the sclera. The choroid provides oxygen and nourishment to the outer
layers of the retina. It also contains a nonreflective pigment that acts as a light shield and prevents
light from scattering. Light enters the front of the eye through a hole in the choroid coat called the
pupil. The iris contracts and dilates to compensate for the changes in light intensity. If the light is
bright the iris then contracts making the pupil smaller, and if the light is dim, the iris dilates
making the pupil bigger. Just posterior to the iris is the lens, which is composed mainly of
proteins called crystallins. The lens is attached by the zonules to the ciliary body that contains the
ciliary muscles that control the shape of the lens for accommodation. Along with the ciliary body
and iris, the choroid forms the uveal tract. The uvea is the middle of the three concentric layers
that make up an eye. The name is possibly a reference to its almost black color, wrinkled
appearance and grape-like size and shape when stripped intact from a cadaveric eye.

88 | Human Physiology