52 



PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



by two roots on each side, one from the optic lobe and 

 one from the thalamus. These quickly unite to form 

 one root on each side, and these join to form the 

 optic chiasm, which lies on the sella turcica (Turkish 

 seat). This is all within the skull (see Figs. 17 and 33). 

 The chiasm immediately separates again into two large 

 nerves, the optic nerves proper, which, piercing the skull 



Fig. 34. — Diagram showing side view of brain and medulla somewhat 

 drawn out to separate the origin of the nerves. The last one (i sfi) is 

 the first spinal. 



at the bottom of the eye sockets, pass forward to enter 

 the eyeball, and there form the retina. Its function is, 

 of course, to respond to impressions of light. 



3, 4, and 6. Oculi Motores. — These we take together 

 because they all have a somewhat similar function, viz., 

 the movements of the eyeball. They come out from the 



