462 THE SENSORIAL FUNCTIONS. 



blood vessels, for supplying nourishment to tlie 

 eye. It has on its inner surface a layer of a dark 

 coloured viscid secretion, known by the name of 

 the Pigmeiitum nigrum^ or black pigment. Its 

 use is to absorb all the light which may happen 

 to be irregularly scattered through the eye, in 

 consequence of reflection from different quarters ; 

 and it serves, therefore, the same purpose as the 

 black paint with which the inside of optical in- 

 struments, such as telescopes, microscopes, and 

 camerae obscurae, is darkened. Within the pig- 

 mentum nigrum, and almost in immediate con- 

 tact with it*, the Retina (r) is expanded, form- 

 ing an exceedingly thin and delicate layer of 

 nervous matter, supported by a fine membrane. 



More than three-fourths of the globe of the 

 eye are filled with the vitreous humour (y), which 

 has the appearance of a pellucid and elastic 

 jelly, contained in an exceedingly delicate tex- 

 ture of cellular substance. The Crystalline 

 humour, (l) which has the shape of a double 

 convex lens, is formed of a denser material than 

 any of the other humours, and occupies the fore- 

 part of the globe of the eye, immediately in front 

 of the vitreous humour, which is there hollowed 

 to receive it. The space which intervenes be- 



* Between the pigmentum and the retina there is found a very 

 fine membrane, discovered by Dr. Jacobson : its use has not 

 been ascertained. 



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