540 



THE EYE IN EVOLUTION 



functions. The same pattern is retained in Fishes in which it has still no 

 frontal connections. In Amphibians there are no fibres from the fore-brain 

 to the thalamus but the earliest phylogenetic evidence of cortical projection 

 occurs in a thalamo-cortical tract to the secondary olfactory cortex, a relay 

 increased in Reptiles in which the dorsal thalamus is large and highly 

 differentiated. In Birds the sensory thalamic nuclei for the first time send 



y^-— — Visual cortex 



;,———— — — . Angular gyrus 



-——— Optic radiations 

 Sensory cortex - — — — -vj V- — ^___ Thalamo-cortical tract 



Sensory tract - 



Xerve II — — — ^ 



Lat. geniculate body{^.^°[^^{ I1~11"' 



Brachium tecti -- — — — — — — —' 



Med. geniculate body ____, 

 (gang, isthitii) 



Superior colliculus (tectum) 



Inferior colliculus (torus) 



Lat. lemniscus 



VIII. Cochlear 



J Vlll. Vestib. 



- Xerve V 



— Post, root ganglia 



Fig. 71.5. — The Visual Pathways in a Typical Mammal. 



a rich supply of axons to the frontal and occipital areas of the neopallium. 

 In Mammals the thalamus, especially its dorsal portion, becomes of extreme 

 importance, being the chief integrating centre for common sensitivity as 

 well as for sensation; all the thalamic nuclei send copious relays of fibres 

 to the cortex and receive cortical efferents equally copiously, many of them 

 inhibitory in type. 



The correlation of sensory and reflex activities in the thalamic region 

 of the diencephalon still requires much further clarification, but it is apparent 

 that whi!< he coordination of the relatively simple movements of the lower 

 Vertebral i akes place in the hind- and mid-brain, the thalamus assumes 



