REFRACTING MEDIA OF EYE 341 



farther away, a point will be found where it can no longer be seen 

 clearly. This is the far point (punctum remotum). The distance 

 between these points is the measure of the range of accommodation. 



RESUME. 



The sclera is protective; the cornea is protective and refractive. 

 The aqueous humor preserves the shape of the cornea and flexibility 

 of the iris and also refracts rays of light. The iris regulates the 

 amount of light admitted, contracting in strong light and when 

 viewing near objects. It is relaxed and inactive in the absence 

 of light. (An active dilatation is caused in certain conditions 

 through stimulation of the radiating fibers.) 



The crystalline lens refracts the rays of light which enter the 

 eye. 



The ciliary muscle, by drawing the suspensory ligament for ward, 

 releases the lens from pressure, so that it becomes more convex 

 and accommodates light-rays from near objects. (Ordinarily, 

 the lens, prevented by the ligament from assuming its greatest 

 convexity, is in the position to transmit light from more distant 

 objects to the retina.) 



The vitreous body preserves the shape of the globe and is also 

 refractive. 



The choroid and iris (meal tract) constitute the dark chamber of 

 the eye. 



The retina is the sensitive nerve layer. 



APPENDAGES OF THE EYE 



The eyebrows, resting upon the superciliary ridges, or eleva- 

 tions caused by the frontal sinuses (p. 21), serve to extend the 

 protection given by the orbit. 



The eyelids (or palpebrae,) attached to the margin of the orbits 

 are necessary for the protection of the eye. They have five layers, 

 skin, smooth and thin; fascia thin and delicate; muscle the 

 palpebral portion of the orbicular muscle; fibrous containing a 

 stiff plate of connective tissue, the tarsal plate; and mucous the 

 layer which lines the lid (conjunctiva). 



The conjunctiva (or conjunctival sac) is the sensitive mucous 

 membrane which is attached to the margins of the lids to line them 



