SENSE AREAS AND ASSOCIATION AREAS. 209 



These locations, therefore, particularly the lateral geniculates, 

 must be considered as the primary Optic centers. From these 

 points the path is continued toward the cortex by new neurons 

 whose axons constitute a special bundle, the occipitothalamic 

 radiation, lying in the occipital part of the internal capsule 

 (see Fig. 82, D). A schema representing this course of the 

 optic fibers is given in the accompanying diagram (Fig. 92). 

 According to this schema, the general relations of each occipital 

 lobe to the retinas of the two eyes is such that the right occip- 

 ital cortex represents the cortical center for the two right halves 

 of the retinas, while the left occipital lobe is the center for the 

 two left halves of each retina, a relation that agrees completely 

 with the results of experimental physiology and clinical studies. 



In addition to the fibers described, which may be regarded as 

 the visual fibers proper, there are other fibers in the optic tracts 

 and optic nerves whose physiological value is not entirely clear. 

 The fibers of this kind that have been described are: (1) Inferior 

 or Gudden's commissure. Fibers that pass from one optic tract 

 to the other along the posterior border of the chiasma. These 

 fibers form a commissural band connecting the two internal 

 (or median) geniculate bodies, and possibly also the inferior 

 colliculi. It seems probable that they belong to the central 

 auditory path rather than to the visual system. (2) Fibers 

 passing from the chiasma into the floor of the third ventricle. 

 The further course of these fibers is not clearly known, but it is 

 possible that they make connections with the nuclei of the third 

 nerve. They will be referred to in the section on Vision in con- 

 nection with the light reflex of the iris. (3) A superior com- 

 missure. Several observers have claimed that there is a com- 

 missural band along the anterior margin of the chiasma which 

 connects one optic nerve or retina with the other. 



There are many points in connection with the course of the 

 optic fibers and the physiology of the different parts of the occip- 

 ital cortex which are unknown and require further investigation. 

 Some of these points may be referred to briefly. 



The Amount of Decussation in the Chiasma. According 

 to the schema given above, half of the fibers in each optic nerve 

 decussate in the chiasma. There is, however, no positive proof 

 that the division of the fibers is so symmetrically made. In the 

 lower vertebrates, fishes, amphibia, reptiles, and most birds 

 the crossing is said to be complete, while in the mammalia a certain 

 proportion of the fibers remain in the optic tract of the same side. 

 In a general way, it would appear that the higher the animal is 

 in the scale of development the larger is the number of fibers that 

 do not cross in the' chiasma. At least it is true that a larger num- 

 14 



