IX 



MID- AND INTEE-BEAIN 



493 



9 



between these two retinal zones corresponds to a vertical plane 

 through the t'ovea eentralis or yellow spot of the retina (Fig. 249). 

 Besides these two handles Samelsohn (1882) distinguishes a third 

 -macular bundle that includes the tibres from the macula lutea, 

 the seat of central or direct vision. 

 The macular bundle again divides 

 into two groups of tibres : one 

 direct, which remains on the same 

 side ; one crossed, which decussates 

 in the chiasma and passes to the 

 other side. Fig. 250 shows the 

 position and direction of the three 

 bundles which make up the optic 

 nerve. 



Partial decussation of the fibres 

 of the optic nerves is constant in 

 man and in the ape, but it is not 

 general in the vertebrate series. 

 The existence of the direct bundle 

 seems to be associated with binocu- 

 lar vision, since in animals whose 

 eyes are directed sideways, so that 

 binocular vision is impossible, e.g. 

 in birds and fishes, there is total 

 decussation of the optic fibres. 

 This rule, however, has certain ex- 

 ceptions : in the rabbit, dog, and 

 cat there is a partial crossing, 

 though less than in monkeys and 

 man, but in the mouse and guinea- 

 pig, according to Singer and 

 Miinzer, decussation is complete. 

 And in some birds, e.g. the owl, in 

 which vision is binocular owing 

 to the position of the eyes, de- 

 cussation is still complete. 



As shown in Fig. 250, the 

 chiasma also contains in its pos- 

 terior parts commissural fibres, 

 which are not connected with the 

 optic nerves and eyes, but pass from the internal geniculate body 

 of one side to that opposite ; these constitute Gudden's commissure, 

 the function of which is quite unknown. 



The optic tract runs obliquely backwards, and after passing 

 around the cerebral peduncle divides into two branches of unequal 

 size (Figs. 189, 190, pp. 327, 328). The lateral branch contains all 

 the optic fibres of the direct, crossed, and macular bundles ; a large 



FIG. 248. Horizontal section of left hemi- 

 sphere. (Flechsig.) 1, anterior; 2, pos- 

 terior limb ; 3, genu of internal capsule ; 

 4, nucleus lenticularis ; 5, nucleus cauda- 

 tus ; 6, optic thalamus ; 7, anterior horn 

 of lateral ventricle; 8, its posterior or 

 occipital horn ; 9, septum lucidum and its 

 central cavity ; 10, 11, fornix ; 10', corpus 

 callosum ; 12, claustrum ; 13, external 

 capsule; 14, insula ; 15, Sylviuu tissuif. 



