THE MESENCEPHALON 453 



corresponding inferior colliculus and disappears from view, 

 under cover of the medial geniculate body. Many of the 

 fibres of which it is composed pass upwards towards the higher 

 parts of the brain in the tegmental portion of the corresponding 

 pedunculus cerebri, but some terminate amidst the cells of the 

 medial geniculate body. 



Each superior brachium passes from the side of the corre- 

 sponding superior colliculus upwards, forwards, and laterally 

 between the medial geniculate body and the pulvinar. A 

 superficial examination of it is sufficient to show that it is 

 connected with the lateral root of the optic tract of the same 

 side by fibres which pass through the interval between the two 

 geniculate bodies ; and with the corresponding lateral genicu- 

 late body. It is important to remember, however, that the 

 fibres of which it is formed are connected with other regions 

 in addition to those indicated by superficial appearances. By 

 means of the fibres which it receives from the optic tract it 

 connects the superior colliculus with the retinae of both sides. 

 Other fibres of the superior brachium connect the superior 

 colliculus with the lateral geniculate body ; and a third series 

 of fibres passes through the superior brachium on its way 

 from the visual region of the occipital part of the cortex to 

 the superior colliculus. 



Tractus Optici. The optic tracts are two relatively broad 

 white strands, right and left, which issue from the correspond- 

 ing postero-lateral angles of the optic chiasma. Each tract 

 consists of fibres derived from the corresponding parts of the 

 retinae of the two sides and of fibres which connect the 

 inferior colliculus of one side with the medial geniculate body 

 of the opposite side. After it issues from the chiasma the 

 tract runs backwards, first round the side of the tuber cinereum 

 and then round the lateral side of the pedunculus cerebri, and, 

 whilst at the side of the pedunculus cerebri, it is in relation, 

 laterally, with the bippocampal gyrus of the cerebrum. When 

 the tract reaches the dorsal part of the lateral aspect of the 

 pedunculus it divides into two portions, which are called its 

 medial and lateral roots. 



The medial root ends in the medial geniculate body and it 

 consists largely, if not entirely, of fibres which connect the 

 medial geniculate body of one side with the inferior colliculus 

 of the opposite side, and which are known as Gudden's 

 commissure. 



