THE RETINA. 305 



14. The inner, nearly transparent, pinkish or whitish coat 

 is the Retina. At this stage of the dissection it has probably 

 become slightly wrinkled, and the white ridges may be seen 

 radiating from the entrance of the optic nerve. 



Drag out the vitreous humor, and note the soft, whitish or 

 pinkish retina ; observe that it is a continuation of the optic 

 nerve. Tear away the retina, noting its consistency. Note 

 the color and luster of the inner surface of the choroid coat. 

 The dark layer on the inside of the choroid coat is the pig- 

 ment-la} 7 er (outer part) of the retina, which adheres to the 

 choroid, and is torn loose from the rest of the retina. 



The reflection of light from this surface of the choroid coat 

 causes the color seen in the eyes of some animals. Turn the 

 remaining coats inside out, and tear the choroid coat from 

 the sclerotic. Observe the blood tubes passing from one to 

 the other. 



The Retina. The chief thing in the eye is the retina. 

 Without this all else is useless. If light of sufficient strength 

 falls on the retina.it stimulates elements in the outer layer 

 (rods and cones), and the nerve impulses, thus started, pass 

 along the fibers of the optic nerve to the brain, and we have 

 the sensation of light. But in order to see anything distinctly, 

 the light must fall on the retina in such a way as to form 

 a distinct image of that object. If the lens be removed, or 

 becomes opaque, as in "cataract/' we fail to see distinctly, 

 though we may discern light from darkness. The other parts 

 of the eye exist to form images on the retina. The cornea, 

 lens, and the aqueous and vitreous humors are the parts 

 directly concerned in forming the images. Light from an 

 object passes through the cornea, aqueous humor, lens, and 

 vitreous humor, and the rays are so refracted as to form an 

 inverted image. If this image falls on a good retina we see 

 well. If these parts do not all work well we see poorly. 



