CENTRAL NERVOUS SYSTEM. 



239 



frontal plane, permits a combination of the fields to a much greater degree. 

 It was in accordance with this principle that partial decussation of these 

 nerves was anticipated by von Gudden in the owl, although the histological 

 evidence for it was not obtained by him. 



The most recent researches on mammals have so increased the number in 

 which a partial decussation occurs that we are justified in regarding this 



Fig. 98.— Illustrating tin- relations or the afferent fibres in the optic nerve. The crossed fibres are 

 indicated by solid lines, the uncrossed fibres by broken lines: AT, nasal side of the right eye ; T, temporal 

 side of the same ; O. E, geniculatum externum ; /'. pulvinar ; C. Q. quadrigeminum anterius. 



arrangement as the rule, although the proportion of the uncrossed fibres is 



small in those mammals in which the eyes are placed laterally. 1 



In man the evidence from degeneration in the optic nerve points to the 

 presence of a crossed and an uncrossed bundle of fibres in each optic nerve, 

 the uncrossed being much the smaller of the two bundles. The contrary 



' Cajal: Die Slruclur des Chiasma opticum nebst einer Allgemeinen Tkeorie der Kreuzung der 



Nervenbahnen, Leipzig, 1899. 



