536 



THE EYE IN EVOLUTION 



the dorsal area of the mid-brain assumes immense importance as a correlating 

 centre for sensory, gravistatic and photostatic impulses ; the fact that these 

 correlations still take place at this level reflects the essentially reflex and 

 instinctive nature of these animals with their poor adaptability and lack 

 of potentiality for further evolution. In Mammals, however, the tectum 

 fails to meet the demands of complex visual differentiations and pluri- 

 sensory combinations, a shift upwards of the sensory centres to a higher level 

 of greater plasticity is necessitated, and this region merely retains the regu- 

 lation of restricted photostatic and other activities. In Mammals the 



Nerve II — j 



Brachiuni tecti __._AX— -4 



Lat. geniculate body ______ 



TECTUM 



Isthmo-tectal tract 



Nucleus lentiformis 



Torus seniicircularis 



Ganglion Isthmi 



Lat. lemniscus 



Nerve VIII 



Nerve V 



"Post, root ganglia 



Fig. 711. — The Visual Pathways in a Teleostean Fish. 



anterior colliculi and tectum are therefore much reduced owing to the 

 diversion of the mass of optic fibres to the lateral geniculate body. In the 

 same manner, in the quest for more ample and effectual sensory associations, 

 the inferior colliculi cease to be an end-station for hearing and serve merely 

 as a relay-station to the cerebral cortex, but nevertheless they retain 

 considerable importance in gravistasis as the main end-station of the lateral 

 lemniscus (Nerve VIII) and the spino- and bulbo-mesencephalic fibres. 



Destruction of the optic lobes in Fish and Amphibia is said to leave the vision 

 normal provided the rest of the mid-brain is intact (Loeser, 1905) ; but in Birds, 

 removal of the colliculi disturbs visual reflexes and produces virtual blindness (Marquis, 

 1935). On the other hand, in Mammals in which the visvxal fibres are relayed to the 

 cortex, lesions of the colliculi or tectum give rise to no observable visual defect (rabbit, 

 rat — Ghisolii, 1937) ; such a lesion, moreover, does not affect the pupillary reflexes 

 which are • iitred in the pretectal area (cat — Keller and Stewart, 1932 ; Magoun, 

 1935). 



