830 THE SENSE OF SIGHT 



This gradual restriction of the range of accommodation is generally 

 explained by saying that the lens loses its elasticity with advancing 

 years. While this is true, the same may be said regarding the struc- 

 tures of the zonula Zinnii and the constituents of the ciliary body. 

 Senescence is a common phenomenon in nature and begins with 

 infancy, although dimmed at this time by the phenomenon of growth. 

 Actual disturbances in vision, however, do not arise until the near 

 point has receded beyond 25 to 30 cm., i.e., until about the forty-fifth 

 year. At this time, most persons experience certain difficulties 

 in accurately focusing small print. This condition is designated as 

 old-sightedness or presbyopia. It indicates that our ciliary mechanism 

 is no longer capable of rendering the lens sufficiently convex to permit 

 us to bring near objects to a precise focus upon the retina. Our ac- 

 commodation for far objects remains of course unimpaired. This 

 difficulty in refraction may be remedied by the employment of a 

 biconvex lens of a strength just sufficient to overcome the senile 

 flatness of the lens. 



Late in life the lens frequently undergoes certain retrogressive 

 changes which lead to an opacity of its substance. When fully 

 developed, this condition, known as cataract, destroys the vision com- 

 pletely, because it prevents the rays of fight from entering the fundus 

 of the eye. The removal of this now useless lens, immediately adjusts 

 the eye for far vision, because it is then wholly dependent for its re- 

 fraction upon the cornea and the aqueous and vitreous humors. All 

 three media together, however, do not equal the refractive power of a 

 normal lens. An eye of this kind may again be converted into a more 

 useful organ by placing a biconvex lens of 10 or 11 diopters in front of 

 it. The same correction must be made for an eye, which has never 

 been in possession of a lens. This inherited condition is known as 

 aphakia. 



This discussion introduces the question of whether the near point 

 or far point, as determined for uniocular vision, remains the same 

 when both eyes are used, as in normal binocular vision. Hess has 

 proved this to be the case whenever the two visual axes are converged 

 in a symmetrical manner, but not when we look laterally outward. 

 The near point of binocular vision is then situated at a somewhat 

 greater distance from the eyes. 



The Innervation of the Ciliary Muscle. — ^Like the sphincter pupillae, 

 the ciliary muscle derives its motor impulses from the anterior part of 

 the nucleus of the oculomotor nerve in the midbrain. These pregan- 

 gfionic fibers are relegated to the cifiary gangfion, where they end in 

 arborizations around other ceHs. Postgangfionically, these fibers are 

 continued as elements of the autonomic system and reach their desti- 

 nation by way of the short, cifiary nerves. This distribution accounts 

 for the close interaction between the cifiary muscle and the constrictor 

 of the pupil; near vision, as has been shown above, being associated 

 with a constriction of the pupil. It also explains the simultaneous con- 



