Section II. CONSTRUCTION

TM 9-258

Section II. CONSTRUCTION

protection, supply it with nutrition, shut out extraneous

3-3. Structure of the Eye.

light, and transmit visual impressions to the brain (fig 3-

a. The eye is an organ nearly spherical in shape,

2). It has a refracting mechanism consisting of the

approximately an inch in diameter which rotates through

crystalline lens, cornea, aqueous humor, and vitreous

quite a wide range in a socket in the bone structure of

humor; together, these form a compound lens. The

the head. Muscles control the eyes in associated

shape of the lens is changed by the ciliary muscles which

movements to a considerable extent and in individual

serve to change the focus of the lens. The iris serves as

movement to a lesser degree. Protection and shade are

a diaphragm to regulate the amount of incoming light

furnished by the eyelids, lashes, brows, and by the

configuration of bones.

b. The eye has three coats which afford it

Figure 3-2. Cross section of eyeball .

3-2