EFFECT OF THE IMAGES UPON THE KETINA 463 



however, the shape of the crystalline lens may be so 

 changed as to secure this accommodation, and the invol- 

 untary movements of these muscles en- 

 ables the human eye to accommodate itself 

 automatically to near and distant objects. 

 (See Fig. 220.) 



The eye a self-supporting camera. — In 

 aU physical details, then, the eye acts as a 

 camera, and forms inverted images of objects 

 upon the surface of the retina, exactly as the 

 camera does upon the sensitive plate. Being 

 living tissue, however, it requires nourish- 

 ment for its cells, and hence the choroid 

 layer not only serves by its color to pre- 

 vent internal reflection but through its 

 nourishes the tissue cells of the inner eye. 

 layer also contains blood vessels which supply 

 food. 



Effect of the images upon the retina. — The actual sen- 

 sation of sight is located not in the eye but in the occipital 

 lobes of the cerebral hemispheres where the visual impulses 

 of the optic nerves are ultimately received. The optic 

 nerve fibers in fact originate in these regions, passing from 

 here to the anterior corpora quadrigemina (see Fig. 184, 

 page 396), and are continued from here to a junction called 

 the optic commissure. At this point many of the fibers 

 cross, thus connecting the left eye with the right hemisphere, 

 and vice versa. Some, however, do not cross, so that if the 

 center of vision in one of the hemispheres is destroyed, there 

 will stiU be sight in both eyes. This crossing of the main 

 fibers is called the optic chiasma. After passing through 

 it the fibers inclosed in two right and left trunks (the optic 



Tig. 220 — Dia- 

 gram to illus- 

 trate changes 

 of the lens in 

 accomodation. 



blood vessels 

 The sclerotic 

 it with 



