Lesson 1, Topic 1
In Progress

Anatomy and Physiology of the Eye

April 11, 2024

Anatomy and Physiology of the Eye

Learning Objective: Examine the anatomy and physiology of the eye.

The eye can be divided into the ocular adnexa, the structures that surround and support the function of the eyeball, and the structures of the eye itself. The anatomy of the ocular adnexa and the eyeball will be discussed in the following sections.

Ocular Adnexa

Learning Objective: Describe the anatomy of the ocular adnexa.

Each of our eyes is encased in a protective, bony socket called the orbit. Our binocular vision sends two slightly different images to the brain that produce depth of vision. Within the orbit, a cushion of fatty tissue protects the eyeball. Only about one-sixth of the eye is outside the orbit. The eyelid helps protect the eye from trauma. The eyebrows help keep irritants out of the eyes. The eyelashes line the margins of the eyelids and help trap foreign particles (Figure 16.1).
The conjunctiva is a thin mucous membrane that lines the eyelid. It covers the outside of the eyeball except for the most central portion. The cornea covers the center of the eye. The mucus secreted from the conjunctiva helps keep the eye moist. The eye blinks about every 2 to 3 seconds. The meibomian glands are located along the edge of the eyelids near the lashes. These glands secrete an oil that is part of the eye’s tears. This oil helps prevent tears from drying up too quickly, creating dry eyes. Blinking causes the lacrimal gland, located in the superior outer portion of the upper eyelid to secrete tears. Tears move across the eyes, cleansing and moistening the surface of the eye. Tears drain into the lacrimal canals in the medial corner of the eye. The tears then drain into the nasal cavity through the nasolacrimal duct. This is why, when a person cries, the excess tears ultimately empty into the nose, producing a watery nasal discharge.

FIGURE 16.1  Ocular adnexa. From Herlihy B, Maebius NK: The human body in health and illness, ed 4, Philadelphia, 2011, Saunders.

Eyeball

Learning Objective: Describe the anatomy of the eyeball.

The eyeball consists of three layers. The outermost layer is made up of the white, opaque sclera (SKLAYR a) and the transparent cornea. The sclera is a tough, fibrous lining that protects the entire eyeball lying within the orbit. It is also known as the white of the eye. The transparent cornea covers the exposed one-sixth of the eyeball. The cornea acts like a window that allows light to enter the eye. The cornea also refracts, or bends the direction of light rays, after they enter the eye (Figure 16.2).
The middle layer is made up of the choroid, the iris, and the ciliary body. The iris is the colored portion of the eye. It is doughnut shaped, with an opening in the center called the pupil. The iris contains muscles that regulate the size of the pupil depending on the intensity of light. It becomes smaller in bright light and opens wider in dim light. The ciliary body contains both the ciliary muscle, which regulates the shape of the lens and the ciliary processes, which secrete aqueous humor. The choroid is the posterior portion of the middle layer of the eye. It is the eye’s vascular layer. It contains many blood vessels that supply nutrients to the outer layers of the retina. The choroid also has a brown pigment that absorbs excess light rays that could interfere with vision.
The inner layer of the eye includes the retina in the posterior portion and the lens in the anterior portion. The delicate tissue of the retina is composed of light-sensitive neurons called rods and cones. Rods are highly sensitive to light and can function in dim light. Cones function in bright light and detect color. Rods and cones convert light into nerve impulses. These impulses travel through the optic nerve to the brain, where they are converted into images.
The lens is a transparent, biconvex body that helps focus light after it passes through the cornea. The lens and the ciliary body divide the eye into two cavities. The posterior cavity, which is between the lens and the retina, contains the transparent, gel-like vitreous humor. The vitreous humor maintains the shape of the posterior eyeball. The anterior cavity, between the cornea and the lens, is filled with aqueous humor, which is continuously produced by the ciliary processes. Aqueous humor helps maintain normal pressure within the eye and provides nutrients to the lens and the cornea (Figure 16.2).

FIGURE 16.2  Anatomy of the eye.

16.1

Critical Thinking Application

To help Amy become familiar with the terminology used in ophthalmology, Kim asks her to describe the three layers of the eyeball. How would you respond?

Physiology of the Eye

Learning Objective: Examine the physiology of the eye.

Vision requires light and depends on the proper functioning of all parts of the eye (TABLE 16.1). A visual impulse begins with the passage of light through the cornea, where the light is refracted; it then passes through the aqueous humor and the pupil into the lens. The ciliary muscle adjusts the curvature of the lens to again refract the light rays so that they pass into the retina, triggering the photoreceptor cells of the rods and cones. At this point, the light energy is converted into an electrical impulse, which is sent through the optic nerve to the visual cortex of the occipital lobe of the brain; there, the light impulse is interpreted, and a picture is created.

Table 16.1

Functions of the Major Parts of the Eye

StructureFunction
ScleraExternal protection
CorneaLight refraction
ChoroidBlood supply
IrisLight absorption and regulation of pupil width
Ciliary bodySecretion of vitreous fluid; changes the shape of the lens
LensLight refraction
Retinal layerLight receptor that transforms optic signals into nerve impulses
RodsDistinguish light from dark and perceive shape and movement
ConesColor vision
Central foveaArea of sharpest vision
Macula luteaCenter of the retina; contains the fovea centralis, the area of most highly acute vision
External ocular musclesMove the eyeball
Optic nerveOne of a pair of nerves that transmit visual stimuli to (cranial nerve II) the brain
Lacrimal glandsProduce tears
EyelidProtects eye