Information about the external world is conveyed to the central nervous system from sensory receptors. Chemoreceptor units for the senses of taste and smell were discussed in Chapters 15 and 17, respectively, and the range of mechanoreceptors that mediate the sense of touch in its various components was presented in Chapter 18. This chapter will examine the systems responsible for vision via photoreceptors of the eye and for the senses of equilibrium and hearing that involve mechanoreceptors in the vestibulocochlear apparatus of the ear.
The eye (Figure 23–1) is a complex and highly developed photosensitive organ that analyses the form, intensity, and color of light reflected from objects, providing the sense of sight. The eyes are located in protective areas of the skull, the orbits, which also contain cushions of adipose tissue. Each eyeball includes a tough, fibrous globe to maintain its shape, a system of transparent tissues that refract light to focus the image, a layer of photosensitive cells, and a system of neurons whose function it is to collect, process, and transmit visual information to the brain. Each eye is composed of three concentric tunics or layers (Figure 23–2): a tough external layer consisting of the sclera and the cornea; a more vascular middle layer consisting of the choroid, ciliary body, and iris; and an inner sensory layer, the retina, which consists of an outer pigmented epithelium and an inner retina proper. The photosensitive inner layer of the retina communicates with the cerebrum through the optic nerve on the eye's posterior side; its anterior edge is called the ora serrata (Figure 23–1).
Internal anatomy of the eye.
The sagittal section of an eye shows the inter-relationships among the major ocular structures, the three major layers or tunics of the wall, important regions within those layers and the refractive elements (cornea, lens, and vitreous).
Layers of the eye.
The sagittal view of an eye shows its three major layers or tunics, with the lens in the anterior opening of the vascular layer and retina.
The lens of the eye is a biconvex transparent structure held in place by a circular system of zonular fibers, which extend from the lens into a thickening of the middle layer, the ciliary body, and by close apposition to the vitreous body on its posterior side (Figure 23–1). Partly covering the anterior surface of the lens is an opaque pigmented expansion of the middle layer called the iris. The round hole in the middle of the iris is the pupil (Figure 23–1).
The eye contains two fluid-filled cavities: the anterior chamber, which occupies the space between the cornea and the iris and the posterior chamber, between the iris, ciliary processes, zonular attachments, and lens (Figure 23–1). Interconnected at the pupil, these contain a clear fluid called aqueous humor. The vitreous chamber lies behind the lens and its zonular attachments and is surrounded by the retina. This chamber is filled with a transparent, gelatinous mass of connective tissue called the vitreous body.
Eye formation begins in the early embryo with epithelial optic vesicles bulging bilaterally from the developing forebrain. These elongate and form optic stalks bearing optic cups (Figure 23–3). Interaction between the optic cups and the overlying surface ectoderm causes the latter to invaginate and detach bilaterally, forming the lens vesicles. In the ensuing weeks, head mesenchyme differentiates to form most of the tissue in the eye's three layers and the vitreous, with the ectoderm of the optic cup and optic stalk giving rise to the retina and optic nerve, respectively, and with surface ectoderm contributing to the cornea (Figure 23–3).
Development of eye.
Eyes begin to form early in development when the optic vesicles bulge bilaterally from the diencephalic region of the forebrain (prosencephalon). These grow, remaining connected to the developing brain by the optic stalks, and approach the surface ectoderm. At this point each vesicle folds in on itself to form the inner and outer layers of the optic cup and inducing surface ectoderm to invaginate into the cup as the lens vesicle, which soon detaches from the surface and lies in the opening of the optic cup. Blood vessels, called the hyaloid vessels, grow along the optic stalk, enter the optic cup and grow toward the developing lens. In the following weeks, head mesenchyme associates with the developing optic cup, which is forming the two major layers of the retina. The mesenchymal cells differentiate around the pigmented layer of the developing retina as the iris, ciliary body and choroid of the vascular layer. Other mesenchymal cells give rise to the more external fibrous layer. The hyaloid vessels regress, leaving a space called the hyaloid canal, in the vitreous body. Folds of skin develop features of the eyelids and conjunctiva. The epidermis lining the latter structures develops in continuity with the surface epithelium of the cornea.
The fibrous, external layer of the eyeball protects the more delicate internal structures and provides sites for muscle insertion. (In reference to the eye, the terms "external/outer" and "internal/ inner" refer to structures closer to the eyeball's surface or in its interior, respectively.) The opaque white posterior five-sixths of the external layer is the sclera...