THE BRAIN 



713 



meet. From the optic chiasma the optic tracts pass backwards to 

 form connections on either side with the external geniculate body, 

 the optic thalamus, and the superior corpus quadrigeminum. A 

 band of fibres, known as v. Gudden's commissure, passes posteriorly 

 in the chiasma, connecting the two internal geniculate bodies. These 

 fibres are not connected with vision, but are probably concerned in 

 some way with the process of hearing. 



The inner fibres of each optic nerve decussate in the chiasma in 

 such a way that the fibres of the nasal half of each retina pass to the 

 tract of the opposite side, while the fibres of the temporal part of each 



FIG. 436. DIAGRAM TO SHOW THE PROBABLE COURSE AND RELATIONS OF THE OPTIC 

 FIBRES. (From " Quain's Anatomy.") 



To simplify the diagram, only single neurons are represented as continuing the two 

 nourons from the re.inse in the gemculate and quadrigerninal bodies of eich 

 side with the visual cortex. This must not be taken to imply that the retinal 

 impressions from the two retinae are fused in these intermediate nuclei. 



retina pass to the same side of the brain. It is stated that in man 

 the fibres coming from the macular region of the retina bifurcate, 

 and pass to the external geniculate bodies of both sides of the brain. 



From the cells of the thalamus and external geniculattf"body arises 

 the optic radiation, which passes through the posterior portion of the 

 internal capsule to the occipital cortex, to end in the neighbourhood 

 of the calcarine fissure. This is the tract concerned in vision. 



From the superior corpus quadrigeminum connection is made 

 with the nuclei of the muscles concerned in the movements of the 

 eyes. Fibres pass in the posterior longitudinal bundle to the third, 



