THE EYE. 427 



name from the yellow pigment held in solution within 

 the cell layers. 



2. The optic papilla, or blind spot, is the point of 

 entrance of the optic nerve. It is found a little to 

 the nasal side of the macula lutea. From the center 

 of this papilla the nerve fibers spread out radially to 

 supply the various parts of the retina. The optic 

 fibers lose their medullary sheaths in their passage 

 through the sclera and the choroid, so that the optic 

 nerve at this point becomes suddenly thinner. Be- 



Physiologic excavation. Blood-vessels. 



Layer 0} nerve fibers-. r ^~~~~~~ 

 Inner molecular layer.\ I 

 Inner nuclear layer p -X,l_ 

 Outer molecular layer^---'-- 

 Outer nuclear layer ':.'"; 

 Rods and cones. - 

 Pigment layer. 

 Sclera.... 



Lamina cribrosa. 



Fig. 294. Section through point of entrance of human optic nerve 

 (Bohm and Davidoff). 



cause of this and the fact that the fibers curve radi- 

 ally, there is produced a deep circular depression in 

 this region. At this point the retina is absent, the 

 choroid coat is interrupted, while connective-tissue 

 fibers of the sclera, called the lamina cribrosa, inter- 

 lace and cross the optic fibers. 



3. The ora serrata is that portion of the retina that 

 marks the posterior limit of the ciliary body. At 

 this point there is a rapid diminution of the retinal 

 layers until but two rows of cells remain, the outer 

 one pigmented. The optic fibers and visual cells 



