The rods and cones are the outer segments of photoreceptor cells whose nuclei form the outer nuclear layer of the retina (Figs. 24.10 and 24.11). The light that reaches the photoreceptor cells must first pass through all of the internal layers of the neural retina. The rods and cones are arranged in a palisade manner; therefore, in the light microscope, they appear as vertical striations.
The retina contains approximately 120 million rods and 7 million cones. The rods are about 2 m thick and 50 m long (ranging from about 60 m at the fovea to 40 m peripherally). The cones vary in length from 85 m at the fovea to 25 m at the periphery of the retina.
Functionally, the rods are more sensitive to light and are the receptors used during periods of low light intensity (e.g., at dusk or at night). The rod pigments have a maximum absorption at 496 nm of visual spectrum, and the image provided is one composed of gray tones (a “black and white picture”). In contrast, the cones exist in three classes: L, M, and S (long-, middle-, and short-wavelength sensitive, respectively) that cannot be distinguished morphologically. They are less sensitive to low light but more sensitive to red, green, and blue regions of the visual spectrum. Each class of cones contains a different visual pigment molecule that is activated by the absorption of light at the blue (420 nm), green (531 nm), and red (588 nm) ranges in the color spectrum. Cones provide a visual image composed of color by mixing the appropriate proportion of red, green, and blue light.
segment of cones outer membrane outer segment of cones and rods
FIGURE 24.9 • Schematic drawing of the layers of the retina.
The interrelationship of the neurons is indicated. Light enters the retina and passes through the inner layers of the retina before reaching the photoreceptor cells of the rods and cones that are closely associated with the pigment epithelium.
The specificity of the cones provides a functional basis to explain color blindness. Trichromats, almost 90% of the population, are those people who can mix a given color from impulses generated in all three classes of cones. True color-blind individuals (almost all are male) are dichromats and are believed to have a defect in the red-, green-, or (much less commonly) blue-sensitive cones. They are able to distinguish different colors by matching the impulses generated by any of the two normal classes of cones. In addition, about 6% of the population of trichromats matches colors with an unusual proportion of red and green. These individuals are called anomalous trichromats.
Each rod and cone photoreceptor consists of three parts:  The outer segment of the photoreceptor is roughly cylindrical or conical (hence, the descriptive name rod or cone). This portion of the photoreceptor is intimately related to microvilli projecting from the adjacent pigment epithelial cells.
 The connecting stalk contains a cilium composed of nine peripheral microtubule doublets extending from a basal body. The connecting stalk appears as the constricted region of the cell that joins the inner to the outer segment. In this layer of
FIGURE 24.10 • Photomicrograph of a human retina. On the basis of histologic features that are evident in this micrograph, the retina can be divided into ten layers as indicated on this photomicrograph. Note that Bruch’s membrane (lamina vitrea) separates the inner layer of the vascular coat (choroid) from the pigment epithelium. 440.
region, a thin, tapering process called the calyceal process extends from the distal end of the inner segment to surround the proximal portion of the outer segment (see Fig. 24.11).