SENSE OF SIGHT. 861 



ligament are attached to the outer membrane are also attached the fibres 

 of the ciliary muscle. When the eye is in a condition of rest, and, 

 therefore, adjusted for distant objects, the ciliary muscle is relaxed and 

 the zone of Zinn, by means of its elastic tension, pulls on the edges of 

 the lens-capsule, thereby extending the lens in a radial direction toward 

 its edge, diminishing its thickness and flattening its curvature. When 

 ive want to focus for near objects the zone is drawn forward and inward 

 by the contraction of the ciliary muscle. Its tension, therefore, decreases, 

 and the lens, by means of its elasticity, bulges forward from the release 

 of the tension of the capsule of the lens and so increases in thickness, 

 and its anterior surface becomes more curved. It is, therefore, evident 

 why a strain on the eyes is experienced when looking at near objects, 

 since in a condition of rest of the eye the muscle is relaxed and the eye 

 is adjusted for parallel rays or for viewing distant objects; but when near 

 objects are closely examined a prolonged muscular effort is required, and 

 this, like all other muscular exertions, results in fatigue. 



Fig. 383.— Myopic Eye. (Landois.) 



In the normal eye the far point of vision may be placed at an infinite 

 distance, for in the normal eye the degree of refraction of the dioptric 

 media is such that parallel rays of light are brought to a focus on the 

 retina. In myopia, or near-sightedness, parallel rays are not focused on 

 the retina in a condition of rest of the ciliary muscle, but cross within 

 the vitreous humor, and after crossing form a diffused image on the 

 retina. The focal point in the myopic eye for parallel rays falls in front 

 of the retina (Fig. 383). The eyeball is, therefore, too long as compared 

 with the focal length of the refracting media. The near point, on the 

 other hand, or the nearest point at which objects may be distinctly seen, 

 lies abnormally near, and the range of accommodation is diminished. 



On the other hand, it is conceivable that the refracting media of the 

 eye may be such that, without accommodation, parallel rays of light, 

 instead pf being focused on the retina, as in the emmetropic or normal 

 eye, or in front of the retina, as in the myopic eye, will come to a focus 

 behind the retina (Fig. 384). Such a defect is spoken of as hypermetropia, 

 and is due to the fact that the degree of refraction of the media of the 



