220 HUMAN PHYSIOLOGY. 



Accommodation. By accommodation is understood the power which 

 the eye possesses of adjusting itself to vision at different distances. In a 

 normal or emmetropic eye parallel rays of light are brought to a focus on the 

 retina; but divergent rays that is, rays coming from a near luminous point 

 will be brought to a focus behind the retina, provided the refractive media 

 remain the same; as a result, vision would be indistinct, from the formation 

 of diffusion circles. It is impossible to see distinctly, therefore, a near and a 

 distant object at the same time. We must alternately direct the vision from 

 one to the other. A normal eye does not require adjusting for parallel rays; 

 but for divergent rays a change in the eye is necessitated; this is termed 

 accommodation. In the accommodation for near vision the lens becomes 

 more convex, particularly on its anterior surface. The increase in convexity 

 augments its refractive power; the greater the degree of divergence of the 

 rays previous to entering the eye, the greater the increase of convexity of 

 the lens and convergence of the rays after passing through it. By this altera- 

 tion in the shape of the lens we are enabled to focus light rays coming from, 

 and to see distinctly, near as well as distant objects. 



Function of the Ciliary Muscle. Though it is admitted that the change 

 in the convexity of the lens is caused by the contraction of the ciliary muscle 

 and the relaxation of the suspensory ligament, the exact manner in which it 

 does so is not understood. When the eye is in repose, as in distant vision, 

 the suspensory ligament is tense, and the lens possesses that degree of curva- 

 ture necessary for focusing parallel rays. In the voluntary efforts to accom- 

 modate the eye for near vision, the ciliary muscle contracts, the suspensory 

 ligament relaxes, and the lens, inherently elastic, bulges forward and once 

 again focuses the rays upon the retina. It is, therefore, termed the muscle 

 of accommodation, and by its alternate contraction and relaxation the lens 

 is rendered more or less convex, according to the requirements for near and 

 distant vision. 



Range of Accommodation. Parallel rays coming from a luminous point 

 distant not less than 200 feet do not require adjustment; from this point up to 

 infinity no accommodation is required for perfect vision. This is termed 

 the punctum remolum, and indicates the distance to which an object may be 

 removed and yet distinctly seen. If the object be brought nearer to the eye 

 than 200 feet, the accommodative power must come into play; the nearer 

 the object, the more energetic must be the contraction of the ciliary muscle 

 and the consequent increase in the convexity of the lens. At a distance of 

 five inches, however, the power of accommodation reaches its maximum; 

 this is termed the punctum proximum, and indicates the nearest point at which 



