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Comparative Morphology of Chordates 



530). In vertebrates other than mammals, all the fibers of each nerve 

 cross those of the other — "complete decussation" — so that each 

 eye is connected only to the visual centers which are on the side of the 

 brain opposite to it. In mammals there are varying degrees of incom- 

 plete decussation. In Ornithorhynchus the crossing is said to be com- 

 plete. There is apparently a correlation between direction of vision and 

 degree of incompleteness of decussation. In mammals with laterally 

 directed eyes (e.g., rodents), the crossing is nearly or quite complete. 

 In carnivores and primates, whose eyes are directed more or less nearly 

 forward, a third (in cat) to a half (in man) of the fibers continue back 

 to visual centers on the same side as the eye from which they came. It 



Fig. 531. The 12 cranial nerves shown as if projected upon a median section 

 of the head. (I) Olfactory lobe; (II) optic; (III) oculomotor; (IV) trochlearis; (V) 

 trigeminus; (VI) abducens; (VII) facialis; (VIII) acusticus; (IX) glossopharyn 

 geus; (X) vagus; (XI) accessorius; (XII) hypoglossus. (Courtesy, Neal and Rand: 

 "Chordate Anatomy," Philadelphia, The Blakiston Company.) 



