506 THE SENSE OF SIGHT. 



the anterior part of the medium next to be described, viz., the 

 vitreous humor, is seated a doubly- 

 FlG - 183 - convex body, the crystalline lens, 



which is the most important refract- 

 ing structure of the eye. The struc- 

 ture of the lens is very complex. It 

 consists essentially of fibres united 

 side by side to each other, and ar- 

 ranged together in very numerous 

 laminae, which are so placed upon 

 one another, that when hardened in 

 spirit the lens splits into three por- 

 tions, in the form of sectors, each of 



Laminated structure of the , . ; IP 



crystalline Ions f -The lamin* whlch 1S . Composed of superimposed 

 are split up after hardening in Concentric laming. The 1DS Ill- 

 alcohol, creases in density and, consequently, 

 in power of refraction, from without 



inwards ; the central part, usually termed the nucleus, being 

 the most dense. The density of the lens increases with age ; 

 it is comparatively 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 humor 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 consists 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 principal use appears to be that of giv- 

 ing the proper distension to the globe of the eye, and of keep- 

 ing the surface of the retina at a proper distance from the lens. 



As already observed, the space occupied by the aqueous 

 humor is divided into two portions by a vertically-placed mem- 

 branous 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 muscular fibres of the 

 iris have a direction, for the most part, radiating from the cir- 

 cumference towards the pupil ; but as they approach the pu- 

 pillary 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 con- 

 traction of the circular ones, which resemble a kind of sphinc- 

 ter, it is diminished. The object effected by the movements 

 of the iris, is the regulation of the quantity of light transmitted 



