18 METHODS OF MICROSCOPICAL RESEARCH. 



ciliary processes. The small muscular ring sur- 

 rounding the pupil is called the sphincter muscle. 



Now, the principal use of the choroid tunic, or 

 rather the pigmentum nigrum which it contains, is 

 to absorb those rays of light which have passed 

 through the transparent retina, preventing their 

 reflection, which would interfere with the distinct- 

 ness of the image. 



By reference to the plate it will be seen that the 

 choroid tunic, the retina, and sclerotica form the 

 three outer rings, while the centre is ramified by 

 nerve filaments and blood-vessels. 



These nerve filaments and blood-vessels lie in the 

 retina, which really forms a continuation and exten- 

 sion of the optic nerve ; it touches the outer cir- 

 cumference of the iris at the front, and lies open as 

 a cup-shaped disc in the interior of the eye ; it 

 receives the rays of light which have passed in turn 

 through the cornea, aqueous humour, crystalline 

 lens, and vitreous humour, and forms a picture at 

 the focus of these. 



The nerve fibres of the retina are excited 

 probably by a product of the action of the light 

 picture upon the visual purple, and the irritations 

 are transmitted to the brain by the optic nerve, 

 producing the sensation of vision. 



The picture produced upon the retina has been 

 compared with that produced by a photographic 

 lens upon a screen or ground glass ; but it will be 

 seen that the instances are not strictly parallel. 



In the eye the rays falling upon the cornea do 

 not again encounter air, the picture is formed in 

 the highly refractive substance, while in the photo- 

 graphic image air intervenes between the screen 

 and the lens, and between the lenses themselves. 



