IX 



MID- AND INTEE-BEAIN 



493 



between these two retinal zones corresponds to a vertical plane 



through the fovea centralis or yellow spot of the retina (Fig. 249). 



Besides these two bundles Sarnelsohn (1882) distinguishes a third 



macular bundle that includes the fibres from the macula lutea, 



the seat of central or direct vision. 



The macular bundle again divides 



into two groups of fibres : 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 

 Mlinzer, decussation is complete. 

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



Which Vision is binocular Owing FIG. 248. Horizontal section of left hemi 



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, 



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, Sylvian fissure. 



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 



