D, Optic nerve
In this chapter, we shall focus on the histology of two organs of special sense, the eye and ear. Other sense organs—the taste buds and olfactory area—were examined in chapters on the oral cavity and respiratory system, respectively. In addition, sensory receptors in the skin, muscle, and tendon were covered in the chapter on the nervous system.
The eye functions as the receptor organ for the sense of vision. It enables the individual to react to different intensities of light and to distinguish differences in the color and shape of objects. The inner layer of the eye, or retina, contains the receptor cells and is protected and nourished by the two outer coats, the sclera and uvea. At this low magnification, the general organization of the structures within the eye can be identified.
The eye has three major coats of tissue. The outer layer is a region of dense connective tissue called the sclera. This covers the posterior two-thirds of the eye, supports the eye, and serves as an attachment for the extraocular muscles. The anterior third of the scleral layer is modified to form the cornea (A), which permits the passage of light to the interior.
The middle coat of tissue in the eye is the vascular layer, or uvea. This region consists of three distinct subdivisions: the choroid, ciliary body, and iris (B).
The innermost region of the eye is the retina (C), which contains the photoreceptor cells, associated neurons, and supporting cells. This layer is sensitive to light and transmits information to the brain via the optic nerve (D).
The region between the cornea and iris is the anterior chamber, and that area between the iris and the lens (E) is the posterior chamber. These two chambers contain a watery fluid called aqueous humor.
Posterior to the lens is the vitreous body containing vitreous humor, a transparent gel that fills this area but has been washed out of this specimen. External to the eye are the coverings of the cornea, the eyelids (F).