CHAP, in.] 



SIGHT. 



957 



movements are not due to the frontal motor area being in- 

 directly thrown into action, since they appear even after this has 

 been removed ; they are obviously brought about by a separate 

 mechanism. The action of the cortex, moreover, appears not to 

 be limited to producing contractions in these ocular muscles; 

 it may take on the character of inhibition. If the third and 

 fourth nerves be divided on one side, so as to leave the rectus 

 externus of that eye alone available, not only is the opposite eye 

 moved outwards, upon stimulation of the cortex on that side, 

 but the eye of the same side follows it to a certain extent; that 

 is to say the stimulation of the cortex, while it leads to contrac- 

 tion in the opposite rectus externus, inhibits a tonic contraction 

 of the rectus externus of the eye of the same side and so 

 permits that eye to move in the same direction as its fellow. 

 This is a further indication of the complexity of the coor- 

 dination of these ocular muscles. This coordination is not 

 effected or not wholly effected in the cortex, since coordinate 

 movements may be produced by stimulating the fibres leading 

 from the cortex. Possibly some at least of the coordination is 

 effected by help of the anterior corpora quadrigemina, since 

 coordinate ocular movements may be obtained by directly 

 stimulating these bodies. The tract of fibres known as the pos- 

 terior longitudinal bundles, which seems to serve as a tie uniting 

 the several ocular nuclei, probably also plays a part in the 

 matter. 



The Horopter. 



596. When we look at any object we direct to it the visual 

 axes, so that when the -retinal image of the object is small, the 



A 



FIG. 161. DIAGRAM ILLUSTRATING A SIMPLE HOROPTER. 



When the visual axes converge at (7, the images a a of any point A on the 

 circle drawn through C and the nodal points k k, will fall on corresponding points. 



'corresponding' parts of the two retinas, on which the two 

 images of the object fall, lie in their respective foveae ceiitrales. 



