462 THE SENSORIAL FUNCTIONS. 



blood vessels, for supplying nourishment to the 

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

 coloured viscid secretion, known by the name of 

 the Pigmentum 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 (v), 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. 



