THE SENSE OF SIGHT. 



637 



THE SENSE OF SIGHT. 



The eyeball or the organ of vision (fig. 171) consists of a 

 variety of structures which may be thus enumerated : 

 Fig. 171* 



Ciliary muscle- 

 Ciliary process- 

 Canal of Petit- 

 Cornea 

 Anterior chamber 



The sclerotic or outermost coat, envelopes about five- 

 sixths of the eyeball : continuous with it, in front, and 

 occupying the remaining sixth, is the cornea. The cornea 

 and front portion of the sclerotic are covered by mucous 

 membrane, the conjunctiva ; that which covers the front 

 of the cornea being little more than squamous epithelium. 

 Immediately within the sclerotic is the choroid coat, and 

 within the choroid is the retina. The interior of the eye- 

 ball is well-nigh filled by the aqueous and vitreous humours 

 and the crystalline lens ; but also, there is suspended in the 

 interior a contractile and perforated curtain, the iris, for 

 regulating the admission of light, and behind the junction 

 of the sclerotic and cornea is the ciliary muscle, the 

 function of which is to adapt the eye for seeing objects 

 at various distances. 



