300 



PHYSIOLOGY 



CHAP. 



mechanism of accommodation, and agrees perfectly with the 

 common observation that in looking at a distant object the eye 

 (more exactly its musculature) is at rest ; on looking at near 

 objects the eye is at work, and easily becomes fatigued. 



The range of accommodation diminishes regularly with age, 

 as shown by the following table (Donders) : 



This gradual decrease in the range of accommodation depends, 

 according to Helmholtz, not on any gradual alteration of the 

 ciliary muscle, but on a progressive variation in the elasticity of 

 the lens, particularly of the capsule, which is due to age and is 

 physiological in character. During development, and especially 

 in senile involution, the lens progressively thickens, and this 

 thickening and hardening advance regularly from the nucleus 

 to the cortex. The depth of the lens, too, diminishes somewhat 

 with age owing to a slight diminution of curvature on both its sur- 

 faces, while in old age the static refraction of the eye also suffers 

 to a slight extent in comparison with what it was in youth. 



The curtailment of near vision manifested in age by diminished 

 power of accommodation is known by the Aristotelian term 

 presbyopia. This is seen in all eyes, hypermetropic, emmetropic, 

 and myopic. The first, which require greater accommodation, 

 show the effects of presbyopia at an earlier stage ; the last, i.e. 

 " short-sighted " eyes, later and to a negligible extent. According 

 to Donders, all who require convex lenses to read from type 

 before the age of 40 are hypermetropic ; all who can read comfort- 

 ably without spectacles at 50 or 55 are myopic. 



Special devices are employed to measure the near and far 

 points of distinct vision on which range of accommodation de- 

 pends. It is usual in determining the near point to show the 

 subject a book with small letters and to measure the least 

 distance at which it is possible to see letters, syllables, and words 

 distinctly. This method is inaccurate, because it is not easy to 

 state exactly at what point the outlines are sharply perceived, 

 since this depends also on other factors e.g. relative size of 

 letters, difference of luminosity between letters and background, 

 degree of contraction of sphincter of the iris, etc. 



