Photoreception 395 



change the focus of the eye is known as the accommodation range, or sim- 

 ply as the accommodation. 



The accommodation range is as much as 1 5 to 20 diopters* in young chil- 

 dren and has an average value of about 4 diopters at the age of 42 years. 

 This means that young children can focus objects as close as 2 inches from 

 the eye as well as those at infinity. The decrease in accommodation permits 

 adjustment at age 42 only from about 8 inches to infinity. The relaxed nor- 

 mal eye is focused for infinity, and accommodation is accomplished by the 

 elasticity of the lens when the cifiary muscles are contracted. The decrease 

 in focusing power with age is caused by the hardening of the lens, and its 

 consequent failure to become slightly rounder and thicker on contraction of 

 the muscle. This is the normal condition of farsightedness or presbyopia 

 (meaning "old" sight), which is almost an inevitable result of old age. It 

 can readily be corrected by means of spectacles, and when the power of ac- 

 commodation is very low, the individual often resorts to the use of two 

 pairs of spectacles or to bifocals, with one lens for near and one for distant 

 objects. 



The most common abnormalities of vision which are corrected by spec- 

 tacles are: (1) elongated eyeball, resulting in nearsightedness, or myopia, in 

 which the image is focused in front of the retina, (2) short eyeball, resulting 

 in farsightedness or hypermetropia, in which the image tends to be formed 

 behind the retina, and (3) non-uniform curvature of the lens or cornea, 

 resulting in astigmatism, the occurrence of different focal points for beams 

 of light in different planes; e.g., vertical lines may appear blurred when 

 horizontal lines at the same distance are not blurred. 



Structure of the Retina. The structure of the retina is shown diagrammat- 

 ically in Figure 110. In this figure the light enters at the bottom, which is 

 the front of the retina, passes through the layer of nerve fibers, a layer of 

 tertiary or ganglionic neurone cell bodies, a layer of synapses, a layer of sec- 

 ondary neurone cell bodies (the inner nuclear layer), another synaptic layer, 

 and then through the cell bodies (outer nuclear layer) of the rods and cones, 

 which are the primary neurones or sense cells. The portion of the rod or 

 cone corresponding to the dendrite of the primary neurone contains the 

 photosensitive material, rhodopsin in the rods and iodopsin in the cones. Very 

 little light is absorbed by the neuronal layers of the retina, except in the 

 photosensitive material. The light which passes the photosensitive material 

 is absorbed by the choroid. 



The neuronal connections of the retina are complex. They have been 

 worked out in great detail by Polyak,^-^' and some of the types of connec- 

 tions are shown in Figure 110. Some cones are connected indirectly to a 

 nerve fiber (e.g., the far left cone in the diagram), and this fiber receives 

 no impulses from other rods or cones. Such a cone has an "exclusive line" to 

 the brain. Other cones (e.g., the far right specimen) are connected to nerve 

 fibers which may receive impulses which originate in other cones or in rods. 

 Certain cones which are connected to an "exclusive" fiber (e.g., the far left 

 cone) are also connected through secondary neurones (inner nuclear layer) 

 to nerve fibers which receive impulses from several or many rods and cones. 

 These connections are important in the mechanism of color vision. The nerve 



* The diopter is defined as the reciprocal of the focal length in meters. 



