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THE AMERICAN NATURALIST [Vol. XLI 



It bleaches rapidly in the light, but (unless the pigmented stratum 

 has been removed experimentally) it is soon restored in the dark. 

 Light thus appears to incite chemical processes in the outer seg- 

 ments of the rods. The inner segments are sometimes described 

 as having a longitudinally fibrillar structure in their outer portions. 

 The opposite ends pass rather abruptly into the very slender rod 

 fibers. Each fiber somewhere in its course expands to enclose 

 the nucleus, and finally terminates in a pyriform enlargement. 

 The nucleus in preserved specimens may have its chromatin 

 arranged in a few broad transverse bands. 



Every cone cell consists of a cone, a cone fiber, and a nucleus. 

 The cones like the rods are divisible into outer and inner segments. 

 The outer segment is usually shorter than that of the rod (12 fi) 

 and tapers somewhat to its rounded extremity. It never contams 

 visual purple, but otherwise, as for example in breaking into trans- 

 verse discs, it resembles the outer segment of the rod. The inner 

 cone segment bulges like the body of a flask. It is divided into 

 an outer, longitudinally fibrillar, ellipsoid portion, and an inner 

 contractile myoid portion. The non-contractile ellipsoid is said 

 to become strongly eosinophilic in the dark. Because of the myoid 

 substance the cones, unlike the rods, may alter their length. The 



ntractility is said to be less i 



, and less i 



the latter than in some amphibia and fishes where the myoid 

 segment is reported to shorten from .Id ." to The nuclei are 



found in a mass of protoplasm near the l)ase of the cone; beyond 

 the nucleus the protoplasm forms a cone fiber which is thicker 

 than that of a rod and which ends in a branched and expanded 



The stimuli received bv the outer segments of the rods and 

 cones are transmitted through their fibers to the nerve cells of the 

 retina, and thence to the brain. A single retinal nerve cell receives 

 the stimuli from several rods and cones. 



Since rods and cones are believed to have different relations to 

 the perception of color their distribution in man and other aninuUs 

 should l)e significant. In the peripheral portion of the human 



where vision is nH»t aciiic, ](><1> and tones are equally abundant, 



