COORDINATION AND CONTROL: (1) THE NERVOUS SYSTEM 



111 



nary nerve cell and a rod- or cone-shaped light-sensitive tip. The cones 

 are the organs of vision in bright light and also of color vision. The rods 

 provide a special apparatus for vision in dim light, and their excitation 

 yields only neutral gray sensations. 



The change from cone to rod vision, like that from "slow" to "fast" photo- 

 graphic film, involves a change from a fine- to a coarse-grained mosaic. The cones 

 are not smaller than the rods, but they act individually, while the rods act in 

 large clumps. Each cone connects with an individual nerve fiber leading to the 



muscles to eyeba 



adjuster neurons 



vitreous chamber 



optic nerve 



ciliary 

 muscles 



Fig. 7.4. A section through the eye, with enlarged detail of the retina (the upper portion 

 diagrammatic) . 



brain, while large clusters of rods have but a single nerve fiber. The capacity of 

 rods for image formation is correspondingly coarse. In very dim light only the 

 rods function, the relatively insensitive cones remaining unstimulated. At mod- 

 erately low intensities of light, about 1,000 times the minimum intensity to which 

 the eye responds, the cones begin to function, bringing a dilute sensation of color. 

 Over an intermediate range of intensities rods and cones operate together, but 

 with increasing brightness the cones come to dominate vision. Whether the rods 

 cease to function in bright fight is not known, but they make no perceptible 

 contribution to vision. 



The tips of the rods contain a light-sensitive pigment called visual purple, or 

 rhodopsin, which bleaches upon exposure to light. As we have already seen, it is 

 formed from vitamin A, deficiency in which causes night blindness. The chemical 

 change caused by light in this pigment is apparently the origin of the nerve 



