Three Coats or Tunics.

TM 9-258

c. The clearest and most distinct vision of the

thin jelly-like substance, the vitreous humor, which is

transparent.

response mechanism of the eye is centered in a small

d. Light entering the eye passes successively

area called the macula or yellow spot on the inner coat or

through the cornea, the aqueous-humor, the crystalline

retina and in a still more sensitive smaller spot in the

center of the macula called the fovea or fovea centralis.

lens, the vitreous humor, and falls on the retina.

Sensations of light are received by tiny cones and rods

attached to nerves and distributed over the fovea,

3-5. Refracting Mechanism.

macula, and retina. These sensations are transmitted to

a. The chief refracting surfaces of the eye are

the brain by the optic nerve at the lower rear of the

those of the cornea and the crystalline lens. The cornea

eyeball.

provides the constant portion of the refraction. The

crystalline lens supplies a variable degree of refraction

which permits the eye to focus on near or distant objects.

3-4. Three Coats or Tunics.

a. The outer of the three coats or tunics of the eye

This gives to the eyes the ability to see far or near

objects distinctly and is termed the power of

is the sclera (fig 3-2) to which are attached the six

accommodation.

muscles that hold the eye in place. It is tough, white, and

b. The iris is a diaphragm which contracts and

flexible and is the white portion of the eye normally seen.

dilates to regulate the amount of light entering the eye

The slightly protruding tough transparent portion at the

(fig 3-3). The iris is the variously colored portion of the

center front of the eye, the cornea, is part of this coat.

eye around the pupil. The pupil is the circular opening in

The cornea and the transparent liquid or aqueous humor

the center of the iris. Its size is limited by the contraction

behind it are part of the refracting mechanism of the eye.

The aqueous humor is essentially a weak salt solution.

or dilation of the iris. It appears black because of the

comparative darkness of the inside of the eyeball. The

b. The middle coat, the choroid, is a deep purple

pupil varies in size from 2 to 8 millimeters, depending

layer made up of veins and blood vessels which supplies

upon the illumination. In intense illumination, the iris

nourishment to the eye tissues. The coloring of the

steps the pupil down to about 2 millimeters. In moderate

choroid forms a dark housing and prevents external light

(daytime) illumination, the opening is about 4 or 5

from diffusing into the eye through the walls of the

millimeters. This is considered to be the opening for

eyeball.

maximal acuity or best resolution. In very faint (night)

c. The inmost coat of the eye is the retina, a highly

illumination, the diameter of the pupil is approximately 8

sensitive layer of nerve fibres by means of which visual

millimeters

impressions are transmitted to the brain. The retina is a

part of the response mechanism. The interior of the eye

is filled with a

3-3