512 THE HUMAN BODY. 



ball and therefore outside the optic mound; in its centre the 

 retina is thinner than elsewhere and so a pit (fovea cen- 

 tralis), 18, is formed. This appears black, the thinned 

 retina there allowing the choroid to be seen through it more 

 clearly than elsewhere. In Fig. 143 is represented the left 

 retina as seen from the front, the elliptical darker patch 

 about the centre indicating the yellow spot, and the white circle 

 on one side, the optic mound. The vessels of the retina 

 arise from an artery (17, Fig. 142) which runs in with the 

 optic nerve and from which branches diverge as shown in 

 Fig. 143. 



The Optic Nerves, Commissure, and Tracts. The optic 

 nerves converge to meet in the optic commissure (m, Fig. 

 141), from which the optic tracts pass to the region of the 

 mid brain. They terminate mainly in the anterior corpora 

 quadrigemina (Chap. XII) and in masses of gray nerve matter 

 lying to the outer sides and in front of these, and known as 

 the corpora geniculata. At the commissure (m, Fig. 141) many 

 fibres cross the middle line, so that fibres from each optic nerve 

 are found in both optic tracts. In general, fibres from the 

 right (that is, the outer or temporal) side of the right retina 

 and the right (i.e. nasal) side of the left retina pass on to the 

 brain in the right optic tract; and similarly for the left sides 

 of the two retinas. Catting the right optic nerve, therefore, 

 causes total blindness of the right eye, but cutting of the 

 right optic tract blindness of the right half of each retina 

 (hemianopia). It will later be seen that rays of light cross in' 

 the eye so that objects to the left in space form images on 

 the right sides of the retinas; and vice versa (Figs. 153, 154). 

 Consequently section or extensive disease of the right optic 

 tract causes left hemianopia; that is, blindness to objects on 

 the left of the line of vision. 



The incomplete crossing of the optic nerve-fibres in man 

 is correlated with the fact that his eyes are so placed that 

 part of the field of vision is common to both. In mammals 

 whose eyes are so laterally placed that at any given moment 

 the objects seen by the two eyes are quite different, the cross- 

 ing at the commissure is complete; when the eyes are placed 

 so that some oojects can be seen simultaneously by the two 

 eyes, some fibres cross, and a greater number cross the larger, 

 the common part of the visual fields. Even in man more of 

 the fibres cross than go direct to the same side of the brain. 



