7 i2 TEXT-BOOK OF PHYSIOLOGY 



very much hypertrophied, thus reinforcing the meridional fibers. In 

 myopia, on the contrary, where the accommodative effort is at a mini- 

 mum, the entire muscles possesses less than its average size and 

 development. 



According to Tscherning, a different explanation of the action of the 

 ciliary muscle must be given. Thus, when it contracts, the antero-internal 

 angle, that portion in close relation with the suspensory ligament, recedes 

 and exerts on the ligament a pressure which in turn exerts a traction on the 

 peripheral portions of the anterior surface of the lens, which produces the 

 deformation observed. At 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 peripheral 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 required 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. 1 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. 

 The nearest point up to which the eye is able to form distinct images of 

 objects is called its near point or punctum proximum. This near point in a 

 healthy boy of twelve years will lie at a point situated 7 cm. from the eye, 

 while the same point lies 20 cm. distant in a man of forty years. Of objects 

 which lie nearer than the punctum proximum the eye cannot form distinct 

 images. 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 force expended in the act of accommodation 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, 

 e.g., 20 cm., a parallel direction. A lens, therefore, which has a focal dis- 

 tance of 20 cm. would be a measure of the force expended ; for such a lens 

 placed in front of the crystalline lens,, when in a state of repose, would, 

 with the assistance of the latter, bring diverging rays coming from the near 



1 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. 



