338 ADAPTATIONS TO SPACE AND MOTION 



colors mixed. The (mosaic?) fusion of brightness-patterns in the meso- 

 gennari layers follows, and the total picture of visual space is synthesized 

 through the inter-mesogennari connections in the corpus callosum. 



Speaking against this view is the fact that color vision is so much 

 more readily disturbed than brightness vision, by cortical lesions. The 

 anatomical facts will fit, as well, an alternative hypothesis that it is 

 pattern which is fused in the geniculate, color in the mesogennari layer. 

 In rats and monkeys, consciousness of the brightness patterns, whether 

 already fused there or not, resides in the lateral geniculate. And, the cat, 

 which has no color vision, has almost as complex a geniculate as man — 

 and therefore, in the cat (and hence in man?) its structure cannot be 

 purposed to accomplish color mixture.* In any case, upon the culmi- 

 nation, in man, of a completely equal representation of the two eyes in 

 each side of the brain, consciousness seems to have been made to wait 

 upon intra-hemispheric fusion, and both processes have been pushed up 

 into the cortex insofar as achromatic sensations are concerned. In 

 animals which have totally decussated optic nerves, and hence have no 

 intra-hemispheric fusion to be accomplished, the whole of visual con- 

 sciousness is enabled to sit at a relatively low level (ordinarily the optic 

 tectum — see p. 522) of the central nervous system. 



The Strange Fate of the Median Eyes — One of the conclusions 

 reached above (that the vertebrates have always had single vision in the 

 binocular fields of their lateral eyes, whatever the structure of the optic 

 chiasma) may shed some light on the curious history of the median eyes : 



There are indications, from elasmobranch embryology, that the pro- 

 vertebrates possessed a metameric series of paired visual organs on the 

 roof of the head. Most of them rapidly disappeared as the lateral, 

 ordinary eyes became perfected; but two pairs of dorsal eyes still hung 

 on almost until the cyclostome level of evolution was reached. 



In most modern cyclostomes, two dorsal eyes are present (Fig. 124). 

 They do not represent a pair, however, for they are arranged in tandem 

 with one behind and below the other. Neither is squarely on the mid-line 

 of the head — instead, one appears to join the roof of the diencephalon 

 to one side, the other on the other side, of the sagittal plane. These two 



*Le Gros Clark has recently suggested that the six layers of the primate lateral geniculate 

 (three conneaed with one retina, three with the other) are related to the three fundamental 

 hue-sensations described by the Young-Helmholtz theory (see pp. 91-6). This is hardly 

 possible, since cats, phalangers, and other nocturnal, achromatic mammals also have lam- 

 inated geniculates — sometimes even with odd numbers of layers. 



