The Eye and Vision

The wall of the eye is composed of three layers (Fig. 18-5). The outermost is a tough protective layer, the sclera, commonly called the white of the eye. This layer extends over the front of the eye as the transparent cornea. The middle layer is a vascular layer, the uvea, which consists of the choroid, the ciliary body, and the iris. The iris, by which we assign the color of the eye, is a muscular ring that controls the size of the pupil,

The wall of the eye is composed of three layers (Fig. 18-5). The outermost is a tough protective layer, the sclera, commonly called the white of the eye. This layer extends over the front of the eye as the transparent cornea. The middle layer is a vascular layer, the uvea, which consists of the choroid, the ciliary body, and the iris. The iris, by which we assign the color of the eye, is a muscular ring that controls the size of the pupil,

Layers Cornea
FIGURE 18-5. The eye. (Reprinted with permission from Cohen BJ, Wood DL. Memmler's The Human Body in Health and Disease. 9th Ed. Philadelphia: Lippincott Williams & Wilkins, 2000.)
Lacrimal Apparatus

FIGURE 18-6. Lacrimal apparatus. (Reprinted with permission from Cohen BJ, Wood DL. Memmler's The Human Body in Health and Disease. 9th Ed. Philadelphia: Lippincott Williams & Wilkins, 2000.)

thus regulating the amount of light that enters the eye. The ciliary body contains a muscle that controls the shape of the lens to allow for near and far vision, a process known as accommodation.

The retina is the innermost layer and the actual visual receptor. It consists of specialized cells, rods and cones, which respond to light. The rods function in dim light, have low visual acuity, and do not respond to color. The cones are active in bright light, have high visual acuity, and respond to color. Proper vision requires the refraction (bending) of light rays as they pass through the structures of the eye to focus on a specific point on the retina. The energy generated within the rods and cones is transmitted to the brain by way of the optic nerve (second cranial nerve). Where the optic nerve connects to the retina, there are no rods or cones. This point, at which there is no visual perception, is called the optic disk, or blind spot. In the retina, near the optic nerve, is the fovea, a tiny depression that has a high concentration of cone cells and is the point of greatest visual acuity (sharpness). The fovea is surrounded by a yellowish spot called the macula.

The eye is protected by its position within a bony socket or orbit. It is also protected by the eyelids, eyebrows, eyelashes, and tears. The lacrimal (tear) glands (Fig. 18-6) constantly bathe the eyes with a lubricating fluid that drains into the nose. There is also a protective conjunctiva, a thin membrane that lines the eyelids and covers the anterior portion of the eye.

The eyeball is filled with a jellylike vitreous body (see Fig. 18-5).

Six muscles attached to the outside of each eye coordinate eye movements to achieve convergence, that is, coordinated movement of the eyes so that they both are fixed on the same point.

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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