572 



THE NERVOUS SYSTEM. 



thus these rays be brought to a focus upon the retina when the 

 eye was in a state of accommodative rest. 



Presbyopia, which is sometimes called " old sight/' sometimes 

 " long sight/' is the condition of the eye present in elderly people. 

 In this condition it is difficult to see near objects, although the 

 vision for those at a distance is unaffected. It is usually attrib- 

 uted to a lessened elasticity of the lens, though the ciliary muscle 

 is also less strong ; some writers state that it depends on dimi- 

 nution of the convexity of the cornea. To aid in correcting it 

 convex glasses are used. 



Presbyopia may begin as early in life as the fifteenth year, 

 although it commonly does not until about the fortieth year. 

 The ability to see objects nearer by in advanced age, *when'pre- 

 viously spectacles were required, is explained by an increased 

 refractive power which sometimes occurs under such conditions. 



FIG. 350. Lines for the detection of astigmatism. 



Astigmatism (Fig. 350). In this condition the cornea is usually 

 at fault, its curvature being greater in one meridian than in another, 

 and consequently the rays of light from an object are not all 

 brought to the same focus, and the image, therefore, is not distinct. 

 Astigmatism is regular when the curvature in any given meridian 

 is regular in that meridian, although the meridian may differ in 

 respect to curvature when compared with the one at right angles 

 to it : the cornea is ellipsoidal and not spherical. Astigmatism 

 is irregular when in any given meridian or meridians the curva- 

 ture of the cornea is not an arc of a circle or an ellipse. It 

 is irregular astigmatism which causes the stars to look as though 

 rays projected from them. For the correction of regular astigma- 

 tism glasses are worn which are segments of a cylinder that is, 

 curved in but one direction and are known as " cylindrical " 

 glasses. Irregular astigmatism cannot be corrected by any glasses. 

 The crystalline lens may also be at fault in astigmatism. 



Regular astigmatism is detected by the observation of con- 

 centric rings or radiating lines, as in Fig. 350. In the former 



