Aqueous humor (blue arrows), which is secreted into the posterior chamber, circulates into the anterior chamber and leaves it through the canal of Schlemm (scleral venous sinus).

iris determine eye color. If melanin is present only in the epithelial cells on the iris's posterior surface, more wavelengths of light are scattered by the iris, causing it to look blue or even green. When the same distribution of melanin occurs with denser-than-usual tissue within the body of the iris, it appears gray. When melanin is within the body of the iris as well as in the epithelial covering, the iris appears brown.

The Inner Tunic The inner tunic of the eye consists of the retina (ret'i-nah), which contains the visual receptor cells (photoreceptors). This nearly transparent sheet of tissue is continuous with the optic nerve in the back of the eye and extends forward as the inner lining of the eyeball. It ends just behind the margin of the ciliary body.

The retina is thin and delicate, but its structure is quite complex. It has distinct layers, including pig-mented epithelium, neurons, nerve fibers, and limiting membranes (figs. 12.33 and 12.34).

There are five major groups of retinal neurons. The nerve fibers of three of these groups—the receptor cells, bipolar neurons, and ganglion cells—provide a direct pathway for impulses triggered in the receptors to the optic nerve and brain. The nerve fibers of the other two groups of retinal cells, called horizontal cells and amacrine cells, pass laterally between retinal cells (see fig. 12.33). The horizontal and amacrine cells modify the impulses transmitted on the fibers of the direct pathway.

In the central region of the retina is a yellowish spot called the macula lutea that occupies about 1 square mil limeter. A depression in its center, called the fovea centralis, is in the region of the retina that produces the sharpest vision.

Just medial to the fovea centralis is an area called the optic disk (fig. 12.35). Here the nerve fibers from the retina leave the eye and become parts of the optic nerve. A central artery and vein also pass through at the optic disk. These vessels are continuous with capillary networks of the retina, and together with vessels in the underlying choroid coat, they supply blood to the cells of the inner tunic. Because the optic disk lacks receptor cells, it is commonly referred to as the blind spot of the eye.

The space enclosed by the lens, ciliary body, and retina is the largest compartment of the eye and is called the posterior cavity. It is filled with a transparent, jellylike fluid called vitreous humor, which together with some collagenous fibers comprise the vitreous body. The vitreous body supports the internal structures of the eye and helps maintain its shape.

In summary, light waves entering the eye must pass through the cornea, aqueous humor, lens, vitreous humor, and several layers of the retina before they reach the photoreceptors (see fig. 12.33). Table 12.6 summarizes the layers of the eye.

D Explain the origin of aqueous humor and trace its path through the eye.

^9 How is the size of the pupil regulated? ^9 Describe the structure of the retina.

In dim light

In normal light

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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