538 COMPARATIVE PHYSIOLOGY. 



over, there is also generally, in bad cases. " spasm of accommo- 

 dation" (i. e., of the ciliary muscle), with increased ocular ten- 

 sion, etc. The remedies are, rest of the accommodation mechan- 

 ism and the use of concave glasses. 



3. The opposite defect is hypermetropia. The hypermetropic 

 eye (Fig. 386), being too short, parallel rays are focused be- 

 hind the retina ; hence no distinct image of distant objects can 



Fig. 386 Anomalies of refraction in the hypermetropic eye (after Landois). 



be formed, and they can only be seen clearly by the use of con- 

 vex glasses, except by the strongest efforts at accommodation. 

 When the eye is passive, no objects are seen distinctly beyond 

 a certain distance — i.e., the near point is abnormally distant 

 (eight to eighty inches). The defect is to be remedied by the 

 use of convex glasses. 



4. Presbyopia, resulting from the presbyopic eye of the old, is 

 owing to defective focusing power, partly from diminished 

 elasticity (and hence flattening) of the lens, but chiefly, proba- 

 bly, to weakness of the ciliary muscle, so that the changes 

 required in the shape of the lens, that near objects may be dis- 

 tinctly seen, can not be made. The obvious remedy is to aid 

 the weakened refractive power by convex glasses. It is practi- 

 cally important to hear in mind -that, as soon as any of these 

 defects in refractive power (though the same remark applies to 

 all ocular abnormalities) are recognized, the remedy should be 

 at once applied, otherwise complications that may be to a large 

 extent irremediable may ensue. 



VISUAL SENSATIONS. 



We have thus far considered merely what takes place in the 

 eye itself or the physical causes of vision, without reference to 

 those nervous changes which are essential to the perception of 



