THE EYE AND VISION 



207 



dilated. By these means the diameter of the pupil is made to vary 

 from ^rd to ^ n th of an inch, and the quantity of light admitted 

 into the eyeball is regulated. The 

 outer rim of the iris is firmly 

 connected with the external coat 

 of the eye, at the junction of the 

 cornea with the sclerotic coat, by 

 the ciliary ligament. 



The retina is a delicate 

 membrane which lies within the 

 choroid coat. It is only from 

 5 ^jth to -gTjth of an inch thick, 

 and covers the whole of the cho- 

 roid with the exception of the 

 ciliary processes. It consists of an 

 expansion of the fibres of the optic 

 nerve, supported by an extremely 

 delicate connective tissue. 



Close behind the iris is a doubly 

 convex, transparent, solid body 



Fig. 191. Ciliary Processes and 

 Betina as seen from behind. 

 Twice the natural size. 



i, pupil; 2, posterior surface of the ins; 

 3, circular muscle of the iris ; 4, ciliary 

 processes ; 5, portion of the choroid. 



Fig. 192. The Connections of the Cornea, Sclerotic, Iris, Ciliary Muscle, 

 Ciliary Processes, and Lens. 



i, cornea ; 2, conjunctiva ; 3, sclerotic ; 4, aqueous chamber ; 5. iris ; 6, centre of the lens ; 

 7, capsule of the lens ; 8, ciliary muscle ; 9, ciliary process ; 10, choroid ; n, retina (anterior 

 portion) ; 12, vitreous chamber. 



called the crystalline lens. Its front surface, which is less convex 

 than the back, is in contact with the iris ; and a space is enclosed 



