IX 



MID- AM) INTER-BBAIN 



493 



b<-i.w<-<-n these two n-tinal /on<-s corresponds to a vertical plane 



th<- fo\va c-ntrali- or y-ll' tf the retina (Fig. 249). 



these two bundl<-:- s hn (1882) distinguishes a third 



macvlw bundle that include the fibres from the macula lutea, 



: central or direct vision. 

 The macula) 1 bundle a^ain divides 

 into two Croups of fibres: one 

 direct, which remains on the same 

 Bid*- ; on<- crossed, which decussates 

 in the chiasma and passes to the 

 other side. Fig. 250 shows the 



aud direction of the three 



wJjich make up the optic 



Partial decussation of the fibres 

 of th<- optic nerves is constant in 

 man and in the ape, but it is not 

 ^- jx ral in the vertebrate series. 

 Th<- existence of the direct bundle 

 be associated with binocu- 

 lar vision, -iiice in animals whose 

 are directed sideways, so that 

 binocular vision is impossible,^. 

 in birds and fishes, there is total 

 d'-cuHsation of the optic fibres. 

 'I'll is 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 

 Muu/.'-r, dccussation is complete. 

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

 which vision in binocular owing 

 ic position of the eyes, de- 

 cussation is still complete. 



As shown in Fig. 250, the 

 "hiasma also contains in its pos- 

 t'-rinr parts coimnissural fibres, 

 which arc 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, 

 th<- function of which is quite unknown. 



The optic tract runs obliquely backwards, and after passing 



around th<- cerebral peduncle divides into two branches of unequal 



1 89, 190, pp. 327, 328). The lateral branch contains all 



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



FIG. 248. Horizontal mxtion of left hemi- 

 Kphere. (Flech*ig.) 1, anterior; 2, \*m- 

 terior limb ; 3, germ of internal capKule ; 

 4, nuclei!* lenticularut ; 5, rmcleiw cauda- 

 tus ; 6, optic thalamtu* ; 7, anterior horn 

 of lateral ventricle ; 8, it* posterior or 

 occipital horn ; 9, septum luculumand it 

 central cavity ; 10, 11, fornix ; W, corpiw 

 ealloKtirn ; 12, elatwtrum ; 13, external 

 capsule ; 14, iiwula ; 15, Sylvian fiKtsur*. 



