THE NERVOUS SYSTEM M*5 



of the third ventricle. Each extremity is connected anteriorly with 

 the optic nerve, and posteriorly with the optic tract. On either 

 side of the commissure is the locus perforatus anticus. The most 

 of the fibres of the commissure proceed from each retina in the 

 corresponding optic nerve, being afferent or centripetal ; but at 

 the back part of the commissure there are the fibres of the inner 

 roots of the optic tracts, which have no connection with either 

 retina. The decussation of fibres in the commissure is only partial. 

 The fibres which arise in the nasal or inner half of the retina cross 

 and enter the optic tract of the opposite side. The fibres which 

 arise in the temporal or outer half of the retina take no part in the 

 decussation, but pass directly backwards into the optic tract of 

 the same side. 



Occupying the back part of the commissure there are, as stated, 

 some fibres which have no connection with either retina. These 

 fibres constitute the commissure of Gudden. They lie behind the 

 decussating fibres, and represent the fibres of the inner root of the 

 optic tract of each side. They constitute the innermost fibres of 

 each optic tract, and they connect the internal geniculate body of 

 one side with its fellow of the opposite side. 



Summary. — The fibres which arise in the nasal or inner half of one retina 

 cross in the optic commissure, and enter the optic tract of the opposite side. 

 The fibres which arise in the temporal or outer half of one retina pass directly 

 backwards into the optic tract of the same side. The fibres of the inner root 

 of each optic tract cross in the back part of the commissure, and form the 

 commissure of Gudden, the fibres of which have no connection with the 

 optic nerves, but connect the two internal geniculate bodies, right and 

 left. The optic commissure therefore consists of the following groups 

 of fibres : (i) The crossed fibres, which axise in the nasal or inner portion of 

 each retina ; (2) the uncrossed fibres, which arise in the temporal or outer 

 portion of each retina, and occupy the outer part of the commissure ; and 

 (3) the fibres of the commissure of Gudden, which occupy the back part of the 

 commissure. 



The optic tract of each side is a flattened white band which 

 passes backwards from the optic commissure. It cur\'^es round the 

 cms cerebri, and in the region of the posterior extremity of the 

 optic thalamus it divides into two roots, outer and inner. The 

 outer or visual root is the larger of the two. It is chiefly com- 

 posed of afferent fibres, which pass from the retina to the brain ; 

 but it also contains efferent fibres, which pass from the brain to 

 the retina. The afferent fibres are derived from (i) the temporal 

 or outer half of the retina of the same side, and (2) the nasal or 

 inner half of the retina of the opposite side, the latter having crossed 

 in the optic commissure. The fibres of the outer root terminate 

 in the external geniculate body, the pulvinar of the optic thalamus, 

 and the upper quadrigeminal body, reaching the last-named body 

 through the superior brachium. They form arborizations around 

 the cells of these bodies which constitute the terminal nuclei or 

 lower visual centres of the outer or visual root. These lower visual 

 centres are connected with the higher or cortical visual centre by 



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