618 TEXT-BOOK OF PHYSIOLOGY. 



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the same time the postero-external portion of the muscle exerts 

 traction on the chorioid, thus sustaining the vitreous and indirectly 

 the lens. 



The reason for the flattening of the periphery of the lens from 

 zonular compression and the sharpening of the central convexity is 

 to be found in the fact that the convexity of the more solid central 

 portion, the nucleus, is greater than that of the lens itself. Hence it 

 is easily understood why a zonular traction would give rise to periph- 

 eral flattening. 



There is, however, one point which seems difficult to harmonize 

 with Tscherning's view; that is, the fact that during accommodation 

 the lens appears to be slightly tremulous, thus showing relaxation, 

 and not increased tension, of the suspensory ligament. 



Range of Accommodation. It has been stated that rays of 

 light coming from a luminous point situated at any distance beyond 

 65 meters are so nearly parallel that no accommodative effort is re- 

 quired for their focalization. So long as the luminous point remains 

 between infinity and 65 meters, the eye, directed toward it, remains 

 completely relaxed. The point at which the object can be distinctly 

 seen without accommodation is termed the far point or the punctum 

 remotum. This for the normal eye is at a distance of 65 meters or 

 beyond.* If the luminous point gradually approaches the eye from 

 a point 65 meters distant, the accommodative power comes into 

 play and gradually increases until it attains its maximum, which it 

 does when the object arrives at a point 20 cm. from the eye. Within 

 this latter distance, distinct vision in the adult is difficult. That 

 point or distance within which a particular eye can not form distinct 

 images of objects is called its near point or punctum proximum. 

 The distance between the punctum remotum and the punctum proxi- 

 mum is termed the range of accommodation. 



Force of Accommodation. The increase in curvature of the 

 lens necessary to focalize rays when the eye is directed from the far 

 to the near point necessitates the expenditure of energy on the part 

 of the ciliary muscle. The energy expended may be measured by a 

 lens the refracting power of which is such as to enable it to produce 

 the same result that is, to give the diverging rays coming from the 

 near point a parallel direction. A lens, therefore, which has for a 

 near point a focal distance of 20 cm. would be a measure of the 

 force expended, for such a lens placed in front of the natural lens, 

 when in a state of repose, would, with the assistance of the latter, 

 bring diverging rays coming from the near point to a focus on 

 the retina. A lens of this character is said to have a refract- 



* In practical ophthalmic work a point six meters distant is taken as the far point 

 for the reason that the rays at this distance are practically parallel. 



