620 



THE NERVOUS SYSTEM. 



of efferent fibres in the optic tract fibres which take their origin in the brain 

 and end in the retina. These are distinguished from the afferent retinal fibres by 

 their exceeding fineness. 



The fibres of the optic tract end in the superior colliculus, in the lateral 

 geniculate body, and in the pulvinar of the thalamus (Fig. 546). The fibres to the 

 superior colliculus reach it through the superior brachium (p. 586), and for the most 

 part sink into its substance to end in terminal arborisations around its cells. The 

 corpus geniculatum laterale receives the largest contribution of fibres from the 

 optic tract. These partly sink into its interior and partly spread out over its 

 surface. The former enter into the construction of the curved lamellae of white 

 matter which traverse this nuclear mass, and to a large extent end in the gray 

 matter which intervenes between these lamellae. The deep fibres which are not 

 exhausted in this way proceed onwards through the lateral geniculate body and 

 enter the pulvinar. Of the superficial fibres which spread over the surface of the 

 lateral geniculate body some dip into its substance and end there, but the majority 

 are carried over it and enter the stratum zonale of the pulvinar. Most of the fibres 



of the optic tract, which end in the pul- 

 vinar, therefore reach their destination by 

 passing either over or through the lateral 

 geniculate body. 



Cortical Connexions of the Optic 

 Path. The superior colliculus, the lateral 

 geniculate body, and the pulvinar consti- 

 tute the lower visual centres or terminal 

 nuclei of the optic tract. The higher 

 visual centre is placed in the cortex of 

 the occipital region of the cerebral hemi- 

 sphere, and the connexions between this 

 and the lower centres are established by a 

 large strand of fibres which runs in the 

 central white matter of the posterior part 

 of the cerebral hemisphere, and which con- 

 stitutes the optic radiation. The optic 

 radiation is composed both of corticipetal 

 and corticifugal fibres. The former arise 

 as the axons of the cells in the lateral 

 geniculate body and the pulvinar, around 

 which the retinal fibres end, and they 

 terminate in the cortex of the occipital 

 lobe. The corticifugal fibres take origin in 

 the cortex of the occipital lobe and end in 

 the pulvinar and superior quadrigeminal 

 body (Ferrier and Turner). Thus consti- 

 tuted, the optic radiation forms a con- 

 spicuous strand (Figs. 546, p. 619; 552, 

 p. 624 ; 567, p. 638), which, reaching the 

 retrolenticular part of the internal capsule, 

 sweeps backwards into the occipital lobe 

 of the cerebral hemisphere on the lateral side of the posterior horn of the lateral 

 ventricle. Its connexions will be studied more fully at a later stage. 



FIG. 547. DIAGRAM OF THE CENTRAL. CONNEXIONS 

 OF THE OPTIC NERVE AND OPTIC TRACT. 



THE PARTS DERIVED FROM THE TELENCEPHALON. 



CEREBRAL HEMISPHERES. 



The cerebral hemispheres form the largest part of the fully developed brain. 

 When viewed from above they form an ovoid mass, the broadest end of which is 

 directed backwards, and the longest transverse diameter of which will be found in 



