DISEASES OF THE EYE. 399 



it by a fine tissue, which quickly undergoes decomposition after death 

 and forms the liquor Morgagni. 



It is held in its situation by attachments formed from the circumfer- 

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

 brane prolonged from the vascular portion of the retina. It is supported 

 in its position anteriorly by the aqueous humor, and posteriorly by the 

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

 ingly thin membrane at the anterior surface of the lenticular capsule is a 

 part 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 lamellae, which can be 

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

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

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

 supposed to be nourished on the principle of endosmose and exosmose 

 through these serrated edges. The structure of the capsule of the Lens 

 is identical with that of the cornea elastica ; and like that membrane, it 

 rolls up 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 perfect transparency, the Crystalline 

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

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

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



The question of the passage 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 every 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 

 supposed 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 will be hereafter 

 described. 



828. The Iris. 



The Iris is a loose movable curtain floating in the aqueous humor. 

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

 margin only to the sclerotic coat and cornea by the ciliary 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 pupillary 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 radiatiny order of fibres again come into play, and then 



