148 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



chiasm (Fig. 81, page 116) is undoubtedly related in 

 some way with the use of the two eyes as one instru- 

 ment, and therefore with the existence of corresponding 

 points. The fibers of the optic roots partly cross and 

 partly do not cross, as shown in the diagram (Fig. 93). 



Fig. 93. — O O', optic roots ; JVW, optic nerves ; M and L, sections of the 

 two eyes ; c c , central spots ; n n\ the nasal halves, and 1 1\ the tem- 

 poral halves, of the retinae. 



Thus each root supplies both eyes, and conversely each eye 

 is controlled by both sides of the brain. The existence of a 

 true chiasm with fibers crossed in this peculiar way may 

 therefore be taken as evidence of the existence of corre- 

 sponding points and the possession of binocular vision. 



The Two Adjustments of the Eyes. — There are 

 two fundamental adjustments of the eyes in every act 

 of looking, viz., the focal adjustment, or accommoda- 

 tion, and the axial adjustment, or turning the axes so as 

 to converge on the object looked at. The one is neces- 

 sary for distinct vision, the other for single vision. Asso- 

 ciated with these, but far less important, is a third, viz., 

 pupillary contraction. 



Two Kinds of Corresponding Points. — We have 

 already (page 127) spoken of corresponding points, ret- 



