Chap. XIX.] ORGAJ^S OF SPECIAL SENSE. 235 



The choroid, or vascular coat of the eye, is a thin dark-brown 

 membrane lining the inner surface of the sclerotic. It is com- 

 posed of connective tissue, the cells of which are large and 

 filled with pigment, and it contains a close network of blood- 

 vessels. It extends to within a short distance of the cornea, 

 and then is folded inwards and arranged in radiating folds, like 

 a plaited ruffle, around the lens and just behind the edge of 

 the cornea. The choroid coat, properly speaking, terminates 

 anteriorly in the ciliary processes, arranged, as above stated, in 

 a radiating circle round the lens ; but closely connected with 

 the anterior margin of the choroid is the iris. 



The iris (^iris, rainbow) is a coloured, fibro-muscular curtain 

 hanging in front of the lens and behind the cornea. It is 

 attached to the choroid, with which it is practically continuous, 

 and is also connected to the sclerotic and cornea at the point 

 where they join one another. Except for this attachment, it 

 hangs free in the interior of the eyeball. In the middle of the 

 iris is a circular hole — the pupil — through which light is 

 admitted into the eye-chamber. The iris, like the choroid, 

 is composed of connective tissue containing a large number of 

 pigment cells and numerous blood-vessels. It contains in 

 addition two sets of plain muscular fibres. One set forms a flat 

 band round the margin of the pupil, and is called the sphincter 

 or contractor of the jjupil ; the other set consists of radiating 

 fibres converging from the circumference to the centre, and is 

 called the dilator of the pupil. The action of these muscle-fibres 

 is affected by light. Under the influence of a bright light 

 the pupil involuntarily contracts so that less light is admitted 

 into the eye-chamber ; in a dim light the pupil involuntarily 

 dilates to admit as much light as possible. The posterior surface 

 of the iris is covered by a thick layer of pigment-cells designed 

 to darken the curtain and prevent the entrance of light. The 

 anterior surface of the iris is also covered with pigment cells, and 

 it is chiefly these latter which cause the beautiful colours seen in 

 the iris. The different colours of eyes, however, are mainly due 

 to the amount, and not to the colour, of the pigment deposited. 



The retina, the innermost coat of the eyeball, is the most 

 essential part of the organ of sight, since it is the only one 

 directly sensitive to light. The sclerotic is the protective, the 

 choroid the vascular or nutritive, and tlie retina is the visual or 



