608 



PHYSIOLOGY 



backwards, showing that the point of the needle which is in the ciliary 

 processes has been moved forwards (Fig. 271). The loosening of the 

 lens during spasm of accommodation is well shown in rare cases 

 where there is congenital absence of the whole iris. In such cases the 

 shaking of the patient's head is seen to cause an oscillation of the 

 lens within the eyeball. 



During accommodation the increased curvature of the anterior sur- 

 face of the lens causes an approximation of this surface to the cornea, 

 which may be directly observed (cp. Fig. 271), especially in people with 

 somewhat prominent eyes. No change takes place in the intra- 

 ocular pressure, in either aqueous or vitreous cavities, as the result 

 of accommodation. The passage of fluid takes place with such 



a k c ease between the fibres of the suspensory 



^ ligament that a slight movement of the lens 



A mm n 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 

 ev6j ^e 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. 272). In consequence of the 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 

 recedes continuously with advancing age. The range of accom- 

 modation is measured by the strength of the lens which will give to 

 rays coming from the 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 : 



Range of accommodation 

 in dioptres 



. 14 



life. (ALLEN THOMSON.) 

 a, at birth ; &, adult ; 



Age 

 10 



20 

 30 

 40 

 50 

 60 

 70 



10 

 7 



4-5 

 2-5 

 1 

 0-25 



This gradual diminution in the range of accommodation gives 



