THE NERVOUS SYSTEM I4»7 



the strand of fibres forming the optic radiation, the higher visual 

 centre being situated in the cortex of the cuneate and lingual gyri 

 of the mesial surface of the occipital lobe. The inner or com- 

 missural root of the optic tract passes beneath the internal genicu- 

 late body, which represents the nucleus of most of its fibres. As 

 stated, these fibres have no connection with the optic nerve. Having 

 traversed the inner part of the optic tract, they cross in the back 

 part of the optic commissure, behind the decussating fibres, and 

 are continuous with the corresponding fibres of the opposite side. 

 Taese are the fibres which constitute the commissure of Gudden. 



Summary of the Optic Tract. — ^The outermost fita-es of each optic tract form 

 the outer portion of the optic nerve of the same side. Most of the other 

 fibres cross in the optic commissure, and form the inner portion of the optic 

 nerve of the opposite side. The innermost fibres cross in the back part of 

 the optic commissure, and form the commissure of Gudden. 



Optic Radiation. — The strand of fibres which forms the optic 

 or thalamo-occipital radiation of either side establishes a con- 

 nection between the lower visual centres (extemal geniculate body, 

 puhdnar, and upper quadrigeminal body) and the higher or cortical 

 visual centre, which is situated (i) on the mesial surface of the 

 occipital lobe, close to the calcarine fissure in the region of the 

 cuneus and lingual gyrus, and (2) on the adjacent part of the 

 extemal surface of the occipital lobe. The strand passes through 

 the post-lenticular part of the internal capsule, and then passes 

 backwards in the medullary substance of the occipital lobe, lying 

 on the outer side of the posterior comu of the lateral ventricle. 

 Thereafter the fibres pass in a radiating manner to the higher or 

 cortical visual centre. 



The optic radiation consists of afferent or corticipetal and efferent 

 or corticifugal fibres. The corticipetal fibres, for the most part, 

 arise as the axons of the nerve-cells wdthin the pulvinar and extemal 

 geniculate body, which are terminal nuclei of the retinal nerve- 

 fibres, and they end in the higher or cortical visual centre. Some 

 corticipetal fibres arise in the higher or cortical visual centre of the 

 opposite side and cross in the splenimn of the corpus callosum. 

 These fibres are of a commissural character. The corticifugal Jibres 

 arise as the axons of the pyramidal cells of the cortex of the visual 

 area of the occipital lobe, and they terminate in the pulvinar and 

 upper quadrigeminal body. 



The lower visual centres are regarded as being connected with the nuclei 

 of origin of the nerves which supply the ocular muscles, probably through 

 means of the posterior longitudinal bundle. 



Mesencephalon. 



The mesencephalon is composed of the corpora quadrigemina, 

 which form its upper or dorsal portion; the crura cerebri, which 

 form its lower or ventral portion; and the aqueduct of Sylvius, 

 which passes through it from the fourth ventricle below to the third 

 ventricle above. 



