638 



THE SENSES 



fiber layer, figure 451. These blood-vessels, however, are absent from the 

 fovea centralis and reduced in size in the macula lutea, figures 451 and 452. 



FIG. 451. Diagram of the Blood-vessels of the Human Retina. (Leber, after Jaeger.) ans, 

 vns, Superior nasal artery and vein; ats, vts, superior temporal artery and vein; ani, vni, inferior 

 nasal artery and vein; ati, vti, inferior temporal artery and vein; am, vm, macular artery and vein; 

 ane, vme, median artery and vein. 



FIG. 452. Blood-vessels of the Macula Lutea. The part that is totally free from vessels is 



the fovea centralis. 



THE OPTICAL APPARATUS. 



The optical apparatus may be supposed, for the sake of description, to 

 consist of several parts; i, A system of transparent refracting surfaces and 

 media by means of which images of external objects are brought to a focus 

 upon the back of the eye; 2, a sensitive screen, the retina, which is a special- 

 ized sensory apparatus in connection with the terminations of the optic nerve, 

 and capable of being stimulated by luminous objects, and of sending such 

 impressions as to produce in the brain visual sensations. To these main 



