VISION. 413 



paint, with which the inside of optical instruments, 

 such as telescopes, microscopes, and camerae ob- 

 scurae, is darkened. Within the pigmentum nigrum, 

 and almost in immediate contact with it,* the Retina 

 (r) is expanded ; forming 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, con- 

 tained in an exceedingly delicate texture 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, im- 

 mediately in front of the vitreous humour, which is 

 there hollowed to receive it. The space which in- 

 tervenes between the lens and the cornea is filled 

 with a watery secretion (a), called the Aqueous 

 humour. This space is divided into an anterior and 

 a posterior chamber by a flat circular partition (i), 

 termed the Iris. 



The iris has a central perforation (p), called the 

 Pupil; and it is fixed to the edge of the choroid 

 coat, by a white elastic ring (q), called the Ciliary 

 Ligament. The posterior surface of the iris is 

 called the Uvea, and is lined with a dark brown 

 pigment. The structure of the iris is very peculiar, 

 being composed of two layers of contractile fibres ; 

 the one, forming concentric circles ; the other, dis- 

 posed like radii between the outer and inner 



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

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

 ascertained. 



