328 THF INTERNAL ENVIRONMF.NT OF THE BODY Part III 



lisht are brought to a focus sooner than those of red Hght, resulting in a 

 blur of white within a halo of color. All cameras are corrected for this defect 

 by combinations of lenses. In the human eye part of the color error is cor- 

 rected by the yellow tinge of the crystalline lens, actually a color filter that 

 passes rays of certain wave lengths, i.e., visible light, but stops the ultra 

 violet. Persons who have had the crystalline lenses of both eyes removed 

 because of cataracts can see in ultraviolet light which is not possible to 



normal eyes. 



The Iris — Regulation of Light. Too much light spoils the picture on a film 

 or retina. In the eye, excess light is stopped by the iris and reflections are 

 reduced by the black lining of the eyeball provided by the pigmented layer 

 of the retina and the choroid coat. The iris is a curtain containing a set of 

 circular muscle fibers that contract in bright light and decrease the pupil 

 and a set of radial muscle fibers that contract in dim light and enlarge the 

 pupil. Such responses to changes in light intensity require 10 to 30 seconds. 

 Flashlight photographs sometimes show the wide open pupils that did not 

 have time to close. 



The muscles of the iris are controlled by autonomic nerves (Table 16.2). 

 Excitement of the sympathetic system, as in extreme pleasure, dilates the pupil. 

 Certain drugs affect the iris; atropine that dilates it is commonly used during 

 examinations of the eye. 



The value of the iris as a curtain is increased by its content of dark pig- 

 ment. In the white races the front layers of cells of the iris are relatively free 

 from pigment and light passing through them appears blue, paler or deeper 

 depending upon the amount of black in the background. Varying amounts 

 of pigment distributed in the front layers of the iris are the basis for all the 

 varieties of hazel, brown, and black eyes. Absence of pigment lets the blood 

 vessels show, giving the pink eye of the albino. 



The Light Receptors. The retina of man and most vertebrates contains two 

 kinds of light receptor cells, the rods and cones, and many associated neurons 

 (Fig. 17.17). The retina is connected with the brain by the bundle of thou- 

 sands of axons that compose the optic nerve. Its exit from the back of the 

 eyeball is the blind spot on the retina in which there are no receptors (Fig. 

 17.16). 



Each receptor is composed of one part that is much like an ordinary nerve 

 cell and either a rod- or a cone-shaped part that is sensitive to light and con- 

 tains carotenoid pigments. The cone cells are responsible for vision in bright 

 light, for detail, and for color vision. They are distributed over the central 

 region of the retina and in the human eye are most abundant in a minute 

 spot of clearest vision, the fovea (Fig. 17.16). The rod cells are especially 

 equipped for vision in dim light, are insensitive to color and are numerous 

 in the sides and periphery of the retina. Each cone cell is usually connected 



