CHAP. XIX.] OKGANS OF SPECIAL SENSE. 229 



sclerotic, is composed of fibrous tissue, which is both firm and 

 unyielding, but, unlike the sclerotic, it has no colour, and is 

 perfectly transparent: it has been aptly termed the "window 

 of the eye." Both the cornea and the anterior portion of the 

 sclerotic are covered by reflections of the mucous membrane 

 lining the eyelids. This is called the conjunctiva, and, kept 

 well lubricated by the secretions of the eye, gives the eyeball 

 its peculiar shining and glossy aspect. The sclerotic is supplied 

 with very few blood-vessels, and the existence of nerves in it is 

 doubtful ; while the cornea has no blood-vessels, but is well 

 supplied with nerves. 



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, fibre-muscular curtain 

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

 attached at its circumference 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 at its circumference, 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 con- 

 nective 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 pupil ; the other set consists of radiating fibres converg- 

 ing 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 



