Sight, and the Eye. 437 



ence or absence of light, thereby enlarging or diminishing the pupil. 

 Inside of and lining the choroid coat of the eye is the third coat called 

 the Retina. It varies in thickness from ^ to ^ of an inch and con- 

 sists of nine layers as shown in fig. 176. The center of the eyeball is 



C 



FIG. 175. Front of Eye Showing Adjustment of Lens for Far and Near Objects. 

 C. Cornea. L. Lens. Jr. Iris. 



N. Adjustment for Near Objects. /''.Adjustment for Far Objects. 



C/n.^-Ciliary Muscle or Choroid Stretcher, which by contracting pulls forward the 

 Choroid and allows the Lens to thicken. 

 Cp. Ciliary Process of the Choroid. 



Iff 



FIG. 1 76. Section of Retina greatly 

 magnified. ( After Max Shultze.) 



1. Fine Limitary Membrane- the 



FIG. 176. 



surface of Retina. 



2. Layer of Nerve fibres. 



3. Nerve Cells similar to those of the 

 Brain. 



4. Granular Layer; a fine, gray, in- 

 distinct mass. 



5. Granular Layer of round, small 

 grains. 



6. Granular Layer of small grains 

 mixed with fibres. 



7. Outer Granular Layer; like num- 

 ber five. 



8. A Fine Membrane. 



9. Layer of transparent Rods and 

 Cones. 



10. Choroid Coat of black pigment. 



The light strikes on No. 1 and passes 

 through all the layers up to No. 9. The 

 thickness of the whole Retina is from 

 1-160 to 1-80 of an inch. 



filled with a transparent fluid of 

 albuminous matter something 

 like the aqueous humor, but 

 denser. It is bounded in front 

 by the crystalline lens and on the 

 sides and rear by the retina. 

 The aqueous humor is secreted 

 from a thin membrane which 

 lines the cornea and which re- 

 pairs any of its losses. But the 

 vitreous humor is not so re- 

 paired and if lost the eye is per- 

 manently destroyed. The crys- 

 talline lens is made up of a 



series of concentric coats and is more dense than either the fluid in front 

 or that behind it. In the disease called cataract, an opaque matter is in 



