388 



PHYSIOLOGY 



These pass round on each side and can be seen to make connection with the 

 back part of the thalamus, the external geniculate body, and the superior 

 corpus quadrigeminum. Part of the tract, which is sometimes called the 

 mesial root, passes into the internal geniculate body. This part of the tract 

 has probably nothing to do with vision and forms a commissure running 

 in the optic chiasma connecting the internal geniculate bodies of the two 

 sides. The course of the optic fibres is shown in the diagram (Fig. 198). In 

 man and in' some other mammals, e.g. dog, monkey, the nerve fibres decussate 



incompletely in the chiasma. The 

 uncrossed bundle is derived from 

 the outer half of the retina of the 

 same side, whereas the crossed 

 bundle is derived 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 must be remem- 

 bered that vision in man is bin- 

 ocular, 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 thala- 

 mus, the former in special relation 

 with the organ of vision, the 

 latter with the organ of hearing. 



The olfactory bulb is also con- 

 nected by tracts with the thalamic 

 region, probably through the 

 column of the fornix and the 

 bundle of Vicq d'Azyr. Since, 

 however, the chief connections of 



the olfactory lobe are with the more primitive portions of the cerebral hemi- 

 spheres, the olfactory tracts will be more conveniently treated of in 

 connection with the latter. 



THE CEKEBELLAR PATHS. We have already traced out the course of 

 spinal fibres which terminate in the cerebellum. 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 



CORP. GEN. INT. 



FIG. 198. Diagram- 

 matic representa- 

 tion of the optic 

 tracts and their 

 connections. 



(CUNNINGHAM.) 



