THE VITREOUS HUMOUR AJx T D IRIS. 645 



The ciliary muscle is composed of organic muscular fibres, 

 which form a narrow zone around the interior of the eye- 

 ball, near the line of junction of the cornea with the 

 sclerotic, and just behind the outer border of the iris (fig. 

 178). The outermost fibres of this muscle are attached in 

 front to the inner part of the sclerotic and cornea at their 

 line of junction, and, diverging somewhat, are fixed to the 

 ciliary processes, and a small portion of the choroid imme- 

 diately behind them. The inner fibres, immediately within 

 the preceding, form a circular zone around the interior of 

 the eye-ball, outside the ciliary processes. They compose 

 the ring formerly called the ciliary ligament. 



The function of this muscle is to adapt the eye for 

 seeing objects at various distances. The manner in 

 which it effects this object will be considered afterwards 

 (p. 650). 



The contents of the ball of the eye are surrounded and 

 kept in position by the cornea, and the dense, fibrous mem- 

 brane before referred to as the sclerotic, which, besides thus 

 encasing the contents of the eye, serves to give attach- 

 ment to the various muscles by which the movements 

 of the eye-ball are effected. These muscles, and the 

 nerves supplying them, have been already considered 

 (p. 539, etseq.). 



Of the Phenomena of Vision. 



The essential constituents of the optical apparatus of the 

 eye may be thus enumerated : a nervous structure to 

 receive and transmit to the brain the impressions of light; 

 certain refracting media for the purpose of so disposing of 

 the rays of light traversing them as to throw a correct 

 image of an external body on the retina ; a contractile 

 diaphragm with a central aperture for regulating the 

 quantity of light admitted into the eye ; and a contractile 

 structure by which the chief refracting medium shall be so 



