SENSE ORGANS. 



117 



derstand the phenomena of vision we must know some- 

 thing of the general structure and function of the retina. 

 Structure of the Retina. — The second pair of cra- 

 nial nerves, as already seen, arise by fibers partly from 

 the optic lobes and partly from the thalamus. These 

 fibers unite to form the optic roots (r, Fig. 81), which con- 

 verge and unite to form the chiasm (ch). From the chiasm 

 there go out diverging the two optic nerves {n), which 

 enter the eye sockets near the conical point, pass for- 



FlG. 82, — Generalized section of retina, etc.: O, optic nerve; S, sclerotic; 

 c/i, choroid ; J?, retina ; £, baciliary layer ; g, granular and nuclear 

 layer ; /", fibrous layer ; V, vitreous humor ; c y central spot. 



ward through the fatty cushion and between the recti 

 muscles, enter the eyeballs a little to the interior or 

 nasal side of the axis or south pole, pierce the" sclerotic 

 and choroid, and spread to form the innermost lining 

 coat directly in contact with the vitreous humor. As a 

 thin, translucent coat it passes forward almost to the 

 attachment of the lens curtain, forming thus a deep cup- 

 shaped receptive plate (Fig. 33, p. 51). Its greatest 

 thickness at the bottom of the cup is one quarter milli- 

 metre or one one-hundredth inch, and thence thins out 

 to a feather edge on the forward margin of the cup. 



Although so thin, its structure is very complex. In 

 a cross section under a low power of the microscope, it 



