Blood vessels and melanin pigment give the choroid an intense dark brown color. The pigment absorbs scattered and reflected light to minimize glare within the eye. The choroid contains many venous plexuses and layers of capillaries and is firmly attached to the retina (Fig. 24.1b). The anterior rim of the uveal layer continues forward, where it forms the stroma of the ciliary body and iris.
The ciliary body is a ringlike thickening that extends inward just posterior to the level of the corneoscleral junction. Within the ciliary body is the ciliary muscle, a smooth muscle that is responsible for lens accommodation. Contraction of the ciliary muscle changes the shape of the lens, which enables it to bring light rays from different distances to focus on the retina.
The iris is a contractile diaphragm that extends over the anterior surface of the lens. It also contains smooth muscle and melanin-containing pigment cells scattered in the connective tissue. The pupil is the central circular aperture of the iris. It appears black because one looks through the lens toward the heavily pigmented back of the eye. In the process of adaptation, the pupil changes in size to control the amount of light that passes through the lens to reach the retina.
The retina consists of two components: The neural retina and pigment epithelium.
The retina is a thin, delicate layer (Fig. 24.1c) consisting of two components:  The neural retina is an inner layer that contains light-sensitive receptors and complex neuronal networks.  The retinal pigment epithelium (RPE) is an outer layer composed of simple cuboidal melanin-containing cells.
Externally, the retina rests on the choroid; internally, it is associated with the vitreous body. The neural retina consists largely of photoreceptor cells, called retinal rods and cones, and interneurons. Visual information encoded by the rods and cones is sent to the brain via impulses conveyed along the optic nerve.
Chambers of the Eye
The layers of the eye and the lens serve as boundaries for three chambers within the eye.
The chambers of the eye are the following.  The anterior chamber is the space between the cornea and the iris.  The posterior chamber is the space between the posterior surface of the iris and the anterior surface of the lens.  The vitreous chamber is the space between the posterior surface of the lens and the neural retina (Fig. 24.2). The cornea, the anterior and posterior chambers, and their contents constitute the anterior segment of the eye. The
FIGURE 24.2 • Schematic diagram canal of illustrating the internal structures of the human eye. The retina consists of that differ in their function. Note that the photosensitive region of the retina occupies the posterior part of the eye and terminates iris anteriorly along the ora serrata. The nonphotosensitive region of the retina is located anterior to the ora serrata and lines the inner aspect of the ciliary body and the posterior surface of the iris. The other layers of the eyeball as well as the attachment of two of the extraocular muscles to the sclera inferior rectus muscle are also shown.
vitreous chamber, visual retina, RPE, posterior sclera, and Development of the Eye uvea constitute the posterior segment.
To appreciate the unusual structural and functional relationships in the eye, it is helpful to understand how it forms in the
The refractile media components of the eye alter the light embryo.
path to focus it on the retina.
As light rays pass through the components of the eye, they are
The tissues of the eye are derived from neuroectoderm, refracted. Refraction focuses the light rays on the photorecep surface ectoderm, and mesoderm.