Chap. 17 



Sensory cells, 

 reception 



RESPONSIVENESS THE SENSE ORGANS 



329 



Nerve cells, 

 conduction 



I Hill lllkf 



\ f \ jl 



— Pigmented cells 

 -Cone cell. 



has most direct 

 pathway to broin ond 

 gives sharpest vision 



Rod cell 



►Brain 



Optic nerve 



Light comes in here 



Fig. 17.17. Section of the central part of the retina (highly magnified). The 

 retina is composed of four layers of cells: an innermost one of nerve cells, the 

 ganglion cells whose long processes (axons) constitute the optic nerve; the bipolar 

 nerve cells that are the intermediates between the ganglion cells and the sensory 

 cells; the sensory rod cells and cone cells; and heavily pigmented epithelial cells. 

 The rod and cone cells are the receptionists of light and are chemically changed 

 by it. The nerve cells are the conductors of effects of those changes. The pigmented 

 layer is a backstop of light; pigment moves within its cells and into their processes. 

 Why light does not first strike the rod and cone cells is explained in the story of the 

 development of the eye. 



with the brain by a single chain of neurons, whereas whole clusters of rod 

 cells are connected with the brain by a single chain (Fig. 17.17). The acute 

 vision of the cones seems to be related to their direct connection with the 

 brain, and the less vivid vision of the rods to their indirect connection with 

 it. Cones produce a sharp, detailed image; rods produce a soft, indefinite 

 one. In the starlight, we see with the rods, and the cones, which are rela- 

 tively insensitive to light, do not function at all. Cones begin to function 

 when the light is of about 1000 times greater intensity than the smallest 

 amount to which the eye can respond. In the gray dawn, the rods dominate 

 vision and there is no color; as the light increases vision is taken over by 

 the cones and the grass is green again. Every rod contains visual purple 

 (carotenoid), a light-sensitive compound related to vitamin A. When light 

 falls on the rods, its energy breaks the visual purple into visual yellow. If 

 dim light is to be perceived, several rod cells must be affected by it at once. 

 The impulse that is created in the associated neurons then passes over them. 

 In the dark, visual yellow is resynthesized into visual purple and the rod cells 

 are charged for another exposure to light. If one comes out of brilliant light 

 into a darkened room, one is completely blinded for a few minutes because 

 the visual purple in the rod cells has been bleached out by the bright light. 



