THE SENSES 



109 



the dendrites of the terminal neurons. These terminal 

 neurons have their cells in the sixth or outer nuclear layer 

 (6) of the retina, and they pass backwards and end in two 

 special kinds of terminations in the seventh layer of the 

 retina the rods and cones (7). These structures are com- 

 posed of two segments a somewhat barrel-shaped basal piece, 

 and a transparent terminal part which in the rods is cylindri- 

 cal and in the cones is pointed. Over the central spot of the 

 eye there are no rods, but the cones lie side by side, and the 

 other layers of the retina are thinned out. The rods and 

 cones are imbedded in the last or eighth layer of the retina 



FIG. 46. To show how parallel rays are brought to a focus on the 

 retina by refraction at the three surfaces (a), anterior sur- 

 face of the cornea ; (6), anterior surface of the lens ; and 

 (c), posterior surface of the lens. 



the layer of pigment cells, or tapetum nigrum. The 

 retina stops abruptly in front at the ora serrata, but the 

 tapetum nigrum, along with another layer of epithelial cells 

 representing the rest of the retinal structures, is continued 

 forwards over the ciliary processes and over the back of the 

 iris. 



The blood vessels of the retina enter in the middle of the 

 optic nerve, and run out and branch in the anterior layer of 

 the retina. 



The interior of the eye may be examined by the Ophthal- 

 moscope, which consists essentially of a small mirror from 

 which light can be reflected into the back of the eye, 



