644 THE SENSE OF SIGHT. 



part, usually termed the nucleus, being the most dense. 

 The density of the lens increases with age ; it is com- 

 paratively soft in infancy, but very firm in advanced life : 

 it is also more spherical at an early period of life than in 

 old age. 



The vitreous humour constitutes nearly four-fifths of the 

 whole globe of the eye. It fills up the space between the 

 retina and the lens, and its soft jelly-like substance con- 

 sists essentially of numerous layers, formed of delicate, 

 simple membrane, the spaces between which are filled with 

 a watery, pellucid fluid. It probably exercises some share 

 in refracting the rays of light to the retina ; but its princi- 

 pal use appears to be that of giving the proper distension 

 to the globe of the eye, and of keeping the surface of the 

 retina at a proper distance from the lens. 



As already observed, the space occupied by the aqueous 

 humour is divided into two portions by a vertically-placed 

 membranous diaphragm, termed the iris, provided with 

 a central aperture, the pupil, for the transmission of light. 

 The iris is composed of organic muscular fibres imbedded 

 in ordinary fibro- cellular or connective tissue. The mus- 

 cular fibres of the iris have a direction, for the most part, 

 radiating from the circumference towards the pupil ; but 

 as they approach the pupillary margin, they assume a 

 circular direction, and at the very edge form a complete 

 ring. By the contraction of the radiating fibres, the size 

 of the pupil is enlarged : by the contraction of the circular 

 ones, which resemble a kind of sphincter, it is diminished. 

 The object effected by the movements of the iris, is the 

 regulation of the quantity of light transmitted to the 

 retina ; the quantity of which is, cateris paribus, directly 

 proportioned to the size of the pupillary aperture. The 

 posterior surface of the iris is coated with a layer of dark 

 pigment, so that no rays of light can pass to the retina, 

 except such as are admitted through the aperture of the 

 pupil. 



