DISEASES OF THE EVE. o'.>9 



it by a fine tissue, wlikh (luickly underguL-s dei.oni]M.isili(»ii ;;fkT dcalli 

 and forinsi the liquor Morgiigui. 



It is held in its situation by attachiuouts funucd I'luiu the (. irLUiufcr- 

 cucc of its capsule to the zonula ciliaris, which latter is the thin mem- 

 brane i)rolonged from the vascular i)ortion of the retina. It is supported 

 in its jiositiou anteriorly by tlie acjueous humor, and i>ostcriorIy by the 

 vitreous humor, contained within the hyaloid membrane. The exceed- 

 ingly thin membrane at the anterior surface of the lenticular cai)sule is a 

 I'art of the membrane that secretes the aqueous humor. 



In structure the Lens is rather soft externally, but increases in density 

 towards its centre. It is composed of concentric lamelke, which can be 

 easily separated from each other by immersion in boiling water or alcohol. 

 These lamelhe, when separating, split into three portions on the back and 

 front ; and the edge of the line of junction is serrated. The Lens is 

 supi)osed to be nourished on the princi]tle of endosmosc and exosmosu 

 througli tliese serrated edges. The structure of the caj.sule of the Lens 

 is identical witli that of the cornea clastica ; and like that membrane, it 

 rolls uji upon itself, when a portion of it is removed. It retains its 

 transparency when immersed in spirit or macerated. It is composed of 

 chondrine, a substance almost analogous to cartilage. 



From its shape, density, and jierfect transparency, the Crystalline 

 Lens acts on the rays of light entering the eye in the same manner as jin 

 ordinary artificial lens. It is the chief agent in jtroducing the impression 

 of an object on the retina, or expansion of the optic nerve. 



The question of the i)assage and refraction of light through various 

 media is in itself exceedingly interesting; but any detailed consideration 

 of this subject would be foreign to the scope of this work. The reader, 

 if he wishes to pursue the inquiry, will find evciy information in works 

 on the human eye. 



No blood-vessels, nerves, or arteries have as yet been discovered in the 

 lens. Yet we know that it must be organized, because it undergoes the 

 phenomena of growth and morbid changes. The manner in which it is 

 sui»pose<l to be nourished has been stated above. The Lens does not, 

 however, appear to have any diseases of its own; but it suffers from 

 diseases of neighbouring structures in various ways, as Avill be hereafter 

 described. 



828. The Iris. 



The Iris is a louse luovable curtain floating in the aqueous huuKir, 

 lu structure it ijs muscular and vascular. It is attached at its outer 

 margin only to the sclerotic coat and cornea by the ciliarv- ligament. As 

 mentioned above, it divides the space containing the aqueous humor into 

 two portions or chambers. The action of the muscular fibres of the Iris 

 is involuntary. The agent which controls and affects its nerves, and 

 through them its muscles, is Light. In a strong light the pupillar}^ open- 

 ing is more or less diminished through contraction of the circular order 

 of the fibres of the Iris. When the stimulus of light is removed, the 

 muscles of the radiatiii'j order of fibres again come into phty, and then 



