THE STRUCTURE OF THE BRAIN STEM 



437 



from the mesial half of the retina on the other side. The right optic 

 nerve thus carries all the impulses originating in the right eye. The 

 right optic tract carries all the impulses originating from stimuli 

 occurring in the left field of vision. It will be remembered that vision 

 in man is binocular, both retinae being concerned in the perception 

 of each field of vision. The external and internal geniculate bodies 

 may be regarded as extensions of the optic thalamus, the former 

 in special relation with the 

 organ of vision, the latter 

 with the organ of hearing. 



The olfactory bulb is also 

 connected by tracts with the 

 thalamic region, probably 

 through the column of the 

 f ornix and the bundle of Vicq 

 d'Azyr. Since, however, the 

 chief connections of the olfac- 

 tory lobe are with the more 

 primitive portions of the cere- 

 bral hemispheres, the olfac- 

 tory tracts will be more 

 conveniently treated of in 

 connection with the latter. 



THE CEREBELLAR PATHS. 

 We have already traced out 

 the course of spinal fibres 

 which terminate in the cere- 

 bellum. They may be shortly 

 summarised as follows : 



(1) The posterior or direct 

 cerebellar tract, originating in 

 Clarke's column of cells of 

 same side, passing up in the 

 lateral columns and by the 



restiform body into the superior vermis of the middle lobe of the 

 cerebellum. 



(2) The anterior cerebellar tract or tract of Gowers, originating in 

 the grey matter of both sides of the cord and passing in the lateral 

 columns through the lateral part of the medulla and pons, and 

 finally attaining the superior vermis through the superior cerebellar 

 peduncles. 



(3) The posterior columns, ending chiefly in the homolateral 

 posterior column nuclei. From these nuclei, though the great mass of 

 fibres passes into the fillet, a certain number from the nuclei of both 



FIG. 199. Diagram- 

 matic representa- 

 tion of the optic 

 tracts and their 

 connections. 

 (CUNNINGHAM.) 



