Vertebrate eyes perform functions analogous to those performed by cameras, but do so using three roughly spherical, concentric tissue layers either derived from or comparable to the dura mater, the pia-arachnoid, and the CNS (Fig. 17-1). The thick, collagenous outer layer forms the sclera—the white of the eye—and continues anteriorly as the cornea and posteriorly as the dural optic nerve sheath. The middle layer is loose, vascular connective tissue that forms the pigmented choroid that lines the sclera; it continues anteriorly as the vascular core of the ciliary body, the ciliary muscle, and most of the iris. The innermost layer, itself a double layer because of the way the eye develops (THB6 Figure 17-1, p. 416), forms the neural retina (closer to the interior of the eye) and the retinal pigment epithelium (adjacent to the choroid); it continues anteriorly as the double-layered epithelial covering of the ciliary body and the posterior surface of the iris. Suspended inside the eye, and not really part of any of these tissue layers, is the lens.

Figure 17-1 Derivatives of the three tissue layers of the eye.

Collectively, structures derived from these three layers, together with the lens, take care of the functions dealt with by cameras: keeping a photosensitive surface in a stable position, focusing images of objects at different distances onto this surface, regulating the amount of light that reaches the photosensitive surface, and absorbing stray light.

Intraocular Pressure Maintains the Shape of the Eye

The shape of the eye is maintained by having it blown up like a soccer ball. The sclera and cornea provide the tough wall, and the inflation pressure is generated by a fluid secretion-reabsorption system much like the CSF system. The ciliary epithelium secretes a CSF-like aqueous humor into the posterior chamber (the space between the iris and the lens). Just as CSF circulates through the ventricles and subarachnoid space and then filters through arachnoid villi, aqueous humor moves through the pupil, into the anterior chamber (between the iris and cornea), gets filtered into a scleral venous sinus (the canal of Schlemm) near the corneoscleral junction, and from there reaches the venous system. The resistance to flow at the filtration site results in a pressure buildup in the aqueous humor. Because the space behind the lens is filled with gelatinous, incompressible vitreous humor, the pressure in the aqueous humor is transmitted throughout the interior of the eye and the shape of the eye is maintained.

The Cornea and Lens Focus Images on the Retina

There’s a big change in refractive index at the interface between air and the front of the cornea, so this is where most of the focusing happens. The lens contributes less because there’s much less change in refractive index going from aqueous humor to lens or from lens to vitreous humor. The major importance of the lens is in adjusting the focus of the eye during accommodation for near vision (see Fig. 17-9 later in this chapter). Contraction of the ciliary muscle relaxes some of the tension on the capsule suspending the lens, allowing the lens to get fatter and the eye to focus on near objects.

The Iris Affects the Brightness and Quality of the Image Focused on the Retina

The pigmented posterior epithelial layers of the iris prevent light from getting into the eye except through the pupil, so regulating the size of the pupil regulates the amount of light reaching the retina (although neural changes in the retina are much more important for regulating the sensitivity of the eye). The pupillary sphincter, innervated by the oculomotor nerve via the ciliary ganglion, makes the pupil smaller (see Figs. 17-7 and 17-8 later in this chapter). The pupillary dilator, innervated by upper thoracic sympathetics via the superior cervical ganglion, makes the pupil larger.

A System of Barriers Partially Separates the Retina from the Rest of the Body

Another indication that the neural retina is really an outgrowth of the CNS is the way its neurons are protected by a similar three-part barrier system. The endothelial cells of retinal capillaries are zipped up by tight junctions, forming a blood-retinal barrier system in the literal sense. The ciliary epithelium, just like the choroid epithelium, prevents diffusion from the ciliary body into aqueous humor. Finally, the retinal pigment epithelium, in a way analogous to the barrier function of the arachnoid, prevents diffusion from the choroid into the retina.

The Retina Contains Five Major Neuronal Cell Types

Key Concept

Retinal neurons and synapses are arranged in layers.

The job of the retina is to convert patterns of light into trains of action potentials in the optic nerve. It does this using five basic cell types (Fig. 17-2), whose cell bodies are arranged in three layers (outer and inner nuclear layers, ganglion cell layer). Alternating with these three layers of cell bodies are an outer and an inner plexiform layer where the synaptic interactions occur. In the outer plexiform layer, photoreceptor cells (rods and cones) bring visual information in, bipolar cells take it out, and horizontal cells mediate lateral interactions. In the inner plexiform layer bipolar cells bring visual information in, ganglion cells take it out (their axons form the optic nerve), and amacrine cells mediate lateral interactions.