CHAP. XXIII.] OF THE HUMAN BRAIN. 469 



Nerves backwards, behind the Optic Commissure. Each 

 ' tract ' is in contact with and turns round the outer 

 border of the Cerebral Peduncle, becoming flattened as it 

 proceeds. Here each of them comes into relation with 

 two small ganglionic nodules (known respectively as the 

 internal and external ' geniculate bodies ') situated at the 

 posterior extremity of the Thalamus (figs. 168, e, i; 156, 8), 

 contiguous to the adjacent anterior segment of the Quadri- 

 geminal Bodies, with which, as well as with the Thalamus 

 itself, many of its fibres, if not all, come into relation 

 before being continued onwards to certain regions of the 

 cortex of the corresponding Cerebral Hemisphere. 



Although the subject is by no means free from doubt 

 and uncertainty, the weight of evidence seems now most 

 in favour of the view that the ' decussation ' at the Optic 

 Commissure is as complete in Man as it is known to be in 

 lower Vertebrates.* This subject will be again referred 

 to in a subsequent chapter in connection with the question, 

 as to what parts of the Cortex of the Hemispheres are 

 most intimately concerned with Visual Impressions. 



Thus it \vould appear that Olfactory Channels do not 

 decussate at all, and that Optic Channels decussate com- 

 pletely. Yet the crossing of the latter channels takes 

 place outside the substance of the Brain, so that in this 

 respect the arrangement differs from that which will be 

 found to obtain for the next two sensory ' Cranial Nerves,' 

 viz. : the Fifth and the Auditory. 



The position of the Fifth Nerve and its superficial 

 connection with the lateral aspect of the * pons Varolii ' 

 may be seen in (fig. 168, v). Its sensory fibres after 

 passing through the ' Gasserian ' ganglion are gathered 

 together into the ' greater root/ the fibres of which, like 

 those of the ' posterior roots ' of the Spinal Nerves, soon 

 See Terrier, " Functions of Brain," pp. 70 and 166. 



