112 THE INDIVIDUAL ORGANISM 



impulse responsible for the sensation of light. How color vision is caused is un- 

 known. Normal human color vision seems to be compounded of three kinds of 

 responses, and is therefore called trichromatic or three-color vision. The three 

 kinds of response call for at least three kinds of cones differing in their sensitivity 

 to the various parts of the spectrum. Presumably they contain three different 

 light-sensitive pigments, but this is still a matter of surmise. We do know that a 

 cone or rod is excited by light to yield either its maximal response or none at all; 

 like a muscle fiber it follows the " all-or-none " rule. We also know that to produce 

 this effect in a rod — and perhaps in a cone — only a single quantum of light need 

 be absorbed. 



The lens of the eye is well corrected for spherical aberration but contains no 

 correction for chromatic or color aberration. This is in part compensated for by 

 three devices. The first is the yellow lens, which filters out all the ultraviolet and 

 near ultraviolet part of the spectrum in which the color error is greatest. The 

 second is the fact that as rod vision decreases and cone vision increases with 

 brighter light, there is a shift in sensitivity toward the red, with the maximum 

 sensitivity passing from the blue-green (rods) to the yellow-green (cones). Color 

 aberration is less toward the red end of the spectrum. And finally, man (along 

 with apes and monkeys) is peculiar in having a yellow patch on the retina opposite 

 the center of the lens — the macula lutea. Its pigmentation lies as a yellow screen 

 over the light receptors of the central retina, subtending a visual angle of some 

 5 to 10 degrees. The yellow pigment is xanthophyll, common in many plant 

 tissues. This pigment takes up the absorption of light in the violet and blue 

 regions of the spectrum just where absorption by the lens falls off. Thus the 

 macula lutea removes for the central retina the remaining regions of the spectrum 

 for which the color error is high. In effect, the human eye, unable to correct for 

 color aberration, throws away those portions of the spectrum that would cause 

 most difficulty. 



One final feature of the retina requires mention. At the center of the yellow 

 spot is a depression, the fovea, the function of which is to spread apart the light 

 rays by allowing them to fall upon its inclined sides. This permits the rays to be 

 distributed over more cone cells (the only ones present in the whole area of the 

 macula lutea) and thus has the effect of increasing the fineness of "grain" of the 

 retina. The fovea lies at the central point of the visual field, and accounts for the 

 great acuity of vision in this tiny region. Without its aid we would be unable to 

 distinguish such fine detail as is required for reading small print or observing the 

 structure of a cell beneath the microscope. 



The rod and cone neurons make synapses with chains of adjustor 

 neurons within the retina. The last member of each chain gives off an 

 axon that leaves the eyeball by way of the optic (second cranial) nerve 

 and so connects with the midbrain and, through thalamic synapses, with 

 the cerebral cortex. 



Hearing. The ear is a double sense organ. Part of it has to do with 

 hearing, the other part with static sensations. Sounds (air vibrations) 

 cause the tympanic membrane, or eardrum, to vibrate in resonance with 



