VISION. 327 



(x,) which is chiefly made up of a tissue of 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 Pigrnenliim nigritvi, or black pigment. 

 Its use is to absorb all the light which may happen to be ir- 

 regularly scattered througli the eye, in consequence of re- 

 flection from different quarters; and it serves, theiefore, the 

 same purpose as the black paint with which the inside of 

 optical instruments, sucli as telescopes, microscopes, and ca- 

 meraB obscurae, 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 vilreous humour (v,) which has the appearance of 

 a pellucid and elastic jelly, contained in an exceedingly de- 

 licate texture of cellular substance. The Crystalline hu- 

 motcr, (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, immedi- 

 ately in front of the vitreous humour, which is there hol- 

 low^ed to receive it. The space which intervenes between 

 the lens and the cornea is filled with a watery secretion (a,) 

 called the tdqueous /tumour. 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 poste- 

 rior 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, disposed like 

 radii between the outer and inner margin.! When the 



* Between the pigmentum and the retina there is found a very fine mem- 

 brane, discovered by Dr. Jacobson: its use has not been ascertained, 

 .f See Fig. 47, vol. i.p. 105. 



