SENSE OF SIGHT. 193 



bright green, or silvery whiteness, and the dark pigment is 

 peculiar to those animals that see in the brightest light of day. 



Fig. 3. 



The eye, after cutting away the sclerotic coat and cornea, to show 

 the vessels of the choroid coat ; magnified, 



6. The Retina** -This is the third and innermost mem- 

 brane of the eye, and is the expansion of the optic nerve, 

 and the immediate seat of vision. It is a soft, thin, and 

 transparent substance, extending from the optic nerve to the 

 crystalline lens, and lining the choroid coat throughout with- 

 out adhering firmly to it. The retina may be divided into 

 three layers, viz., serous, nervous, and vascular ; the former 

 of which passes on over the surface of the lens, and forms 

 part of its capsule. There is a small portion of the retina, 

 supposed to be near where the optic nerve pierces the 

 sclerotic coat, which is not susceptible of vision, as may 

 be shown by an experiment hereafter mentioned. It was 

 formerly supposed, that the retina was endowed with extreme 

 sensibility ; but it is now ascertained that it is almost insen- 

 sible to every stimulus but that of light. The same is true 



* Retina, a net. 

 17 



