544 THE EYE IN EVOLUTION 



geniculate bodies, while the optic axons again terminate in the tectum and 

 its dorsal extensions, the optic lobes (Fig. 711). Here there is a point-to-point 

 representation of the retina (Lubsen, 1921 ; Buser and Dusardier, 1953). 

 In Amphibians the tectum is more highly differentiated, but still there is 

 no higher projection of visual fibres (Fig. 712) ; in these and in Reptiles 

 optic fibres terminate in the lateral geniculate body which emits only a 

 geniculo-tectal tract (Fig. 713). In Birds the same relations are maintained ; 

 here again a point-to-point representation of the retina has been physio- 

 logically demonstrated (Hamdi and Whitteridge, 1954). In addition to the 

 main end-station in the tectum, however, a bundle of optic fibres in Birds 

 has a thalamic termination in a basal optic ganglion (the ganglion ecto- 

 mammillare of Edinger) (Fig. 714). Near the anterior border of the tectum 

 a dorsal thalamic nucleus (the spiriform nucleus) receives fibres from this 

 basal optic ganglion, in addition to fibres from the large spinal and bulbar 

 tracts and descending fibres from the occipital area of the cortex and corpus 

 striatum ; it has, however, no ascending projection, and the function of 

 this thalamic system is therefore still entirely photostatic. 



In the lower Vertebrates the superior colliculus which has evolved 

 from the optic tectum receives the mass of optic fibres. In Mammals, 

 however, the vast majority of the optic fibres (70 to 80%) terminates in 

 the dorsal nucleus of the lateral geniculate body whence they are relayed 

 by a cortical projection to the occipital cortex ; while, as in the lower 

 Vertebrates, the minority goes either directly to the tectum (superior colli- 

 culus) in the brachium tecti or indirectly after being relayed in the ventral 

 nucleus of the lateral geniculate body. In the colliculus a point-to-point 

 representation from the retina has been reported in the cat, the goat, and 

 the rabbit (Apter, 1945 ; Coopered aZ., 1953 ; Hamdi and Whitteridge, 1953). 

 From this system there is, as always, no cortical projection. The ventral 

 geniculate nucleus is thus phylogenetically the older and corresponds with 

 the entire lateral geniculate body of the lower Vertebrates, decreasing in 

 importance as the visual system swings from a tectal to a cortical orientation. 



It is clear, therefore, that initially the visual system is developed in 

 association with the postural and gravistatic systems in the tectum — the 

 meeting-place of optic, static, tactile, gustatory and proprioceptive impulses, 

 an area which, although it receives fibres from the cortex, sends no fibres to 

 it. It is easy to underestimate the great importance of vision in orientation 

 and equilibration, for in man these static functions are readily overshadowed 

 by the apparent preponderance of the dynamic aspects of vision and the 

 overwhelming importance of its sensory and cognitive functions. The 

 phylogenetic importance of photostasis, however, is obvious. As evolution 

 proceeds, sensory functions assume greater and greater preponderance, and 

 althougl] the reflex and photostatic aspects of vision, which are as complex 

 and elal: \te in Primates as in the lower Vertebrates, are retained in the 

 tectum, I tually in the higher Vertebrates the epicritic visual functions 



