428 



ANATOMY FOR NURSES 



[Chap. XX 



and extends from the entrance of the optic nerve forward to the 

 margin of the pupil. 



The retina is usually described as consisting of eight layers and 

 two limitinii; membranes ; of these layers, three are most impor- 

 tant : — 



(1) Eighth layer, or layer 

 of nerve-fibres, is the internal 

 layer. 



(2) Seventh layer is the 

 layer of nerve-cells. 



(3) First layer, or layer of 

 rods and cones, is the external 

 layer. (See Summary, page 

 440.) 



The fibres of the optic 

 nerve, after piercing the sclera 

 and choroid at the back of 

 the eye, spread out and form 

 the eighth, or innermost, 

 layer of the retina. The fibres 

 then pass, with more or less 

 direct communications, pe- 

 ripherally through the other 

 layers, until they may be said 

 to terminate in the layer of 

 rods and cones. Rays of light 

 produce no efi'ect upon the 

 optic nerve without the in- 

 tervention of the rods and 

 cones, which act as end 

 organs. 

 Blind spot. — The optic 

 ner\e pierces the eyeball not exactly at its most posterior point, 

 but a little to the iimer side. This point where the optic nerve 

 enters is called the blind spot. There are no rods and cones at this 

 spot, and rays of light falling upon it produce no sensation. 



Macula lutea. — There is one point of the retina that is of great 

 importance, and that is the macula lutea, or yellow spot. It is 

 situated about one-twelfth inch (2.08 mm.) to the outer side of the 



Fio. 201. — Diagrammatic bEcrioN of 

 THE Human Retina. 8, layer of nerve- 

 fibreo; 7, layer of nerve-cells; 1, layer of 

 rods and cones. (M. Schultze.) 



