522 ADAPTATIONS TO PHOTIC QUALITY 



controlling extra-ocular, intra-ocular, and skeletal muscles make the optic 

 tectum very much the chief center for visual reflexes. Whatever visual 

 consciousness a fish may have — including the awareness of hues — must 

 reside in the optic tectum. There is no 'higher' visual center in the fish 

 brain. But there are connections of the tectum with other brain regions, 

 some of which might be vital to visual associations, Nolte, however, 

 working with Phoxinus and Gasterosteus, found that the removal of 

 such of these regions as could be destroyed without killing the fish, 

 failed to disturb the learning of associations with color stimuli. He 

 extirpated in turn the cerebral lobes, the habenular ganglia, and the 

 molecular layer of the cerebellum. The fishes still responded to color; 

 and others, which were trained to colors only after such operations, 

 learned in the normal time. 



We might expect that even in the highest vertebrates much of visual 

 consciousness, including perhaps hue-consciousness, would continue to 

 be mediated by the homologues of the optic tectum, which are the 

 superior collicuH. But in the evolution of the nervous system the superior 

 colliculi have become very decidedly a spur track of the visual pathway, 

 and are concerned only with relaying impulses for reflex and willed 

 movement to the extra-ocular and other muscles (see Fig. 123, p. 335). 

 We have seen (p. 336) that in man all visual sensations reside in the 

 cortex, where color sensations are most susceptible of all to injuries of 

 the visual area in the occipital lobe. In man, the lateral geniculate nu- 

 cleus may play a considerable role in vision; but the optic tectum is 

 purely a reflex center and has surrendered, to the geniculate and the 

 cortex, any functions in visual sensation which it may have had in the 

 fishes and amphibians. Although the teleostean and primate color-vision 

 systems may be physiologically identical in their dependence upon three 

 elementary central processes, it would seem that they must be very differ- 

 ently localized in the respective central nervous systems — in the optic 

 tectum in the one, and in the lateral geniculate or in the cerebral cortex 

 in the other. The location of color vision in the brains of reptiles and 

 birds is a problem which has had no attention, though it should be 

 susceptible of experimental attack. 



In the detailed localization of the color-sense within the primate cor- 

 tex, an interesting start was made a decade ago by the Swedish neurol- 

 ogist Henschen. He found that in layer IV of the visual cortex (see pp. 

 334-7 and Fig. 123) two different types of ganglion cells could be seen 

 in species having duplex retinas. Henschen identified these two types as 



