546 PHYSIOLOGY 



be directly observed (cp. Fig. 273), especially in people with somewhat 

 prominent eyes. No change takes place in the intraocular pressure, in either 

 aqueous or vitreous cavities, as the result of accommodation. The passage 

 of fluid takes place with such ease between the fibres of the suspensory 



FIG. 273. Accommodation in the cat's eye. R, distance ; A, for near vision. 



(After BEER.) 



Two needles have been passed through edge of cornea into ciliary bodies, to show 

 forward movement of latter during accommodation. 



ligament that a slight movement of the lens forwards or backwards does 

 not upset the equality of pressures in the two chambers. 



THE RANGE OF ACCOMMODATION. Assuming that, as is the case with 

 the normal eye, the tension of the suspensory ligament is sufficient to keep 

 the eye focused for parallel rays, the near point of vision will be determined 

 by the unconstrained shape of the lens, i.e. the amount by which its curvature 

 can increase when the suspensory ligament is completely relaxed. 



The shape of the lens varies with age, its convexity, being greatest 

 directly after birth and diminishing steadily from that time to the age of 

 sixty or seventy (Fig. 274). In consequence of the 

 c small size of the eyeball, the eye at birth is generally 

 ^ somewhat hypermetropic, but from the age of ten 

 onwards we find that the point of near vision re- 

 cedes continuously with advancing age. The range 



FIG. 274. Lens from human n . J . 



eye at different periods of of accommodation is measured by tne strength 01 



life. (ALLEN THOMSON.) t ^ e i ens wn i cn w {\\ g i ve to rays coming from the 



a, at k^; Mdult ; near point of distinct vision the same direction as if 



they came from the far point. In a normal eye the far 



point is at infinite distance and the rays are parallel. The change in the range 

 of accommodation with advancing years is shown in the following Table : 



Age Range of accommodation in dioptres 



10 ........ 14 



20 ........ 10 



30 ........ 7 



40 ........ 4-5 



50 ........ 2-5 



60 ........ 1 



70 ........ 0-25 



This gradual diminution in the range of accommodation gives rise finally 

 to disturbances of vision which are known as presbyopia. The ordinary reading 



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