246 HUMAN PHYSIOLOGY. 



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 curvature necessary for 

 focusing parallel rays. In the voluntary efforts to accommodate the 

 eye for near vision, the ciliary muscle contracts, the suspensory liga- 

 ment 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 per- 

 fect vision. This is termed the punctum remotum, 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 accom- 

 modative power must come into play; the nearer the object, the more 

 energetic must be the contraction of the ciliary muscle and the cbn- 

 sequent 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 an object may be seen distinctly. The distance between 

 these two points is the range of accommodation. 



Optic Defects. Astigmatism is a condition of the eye which 

 prevents vertical and horizontal lines from being focused at the same 

 time, and is due to a greater curvature of the cornea in one meridian 

 than in another. 



Spheric aberration is a condition in which there is an indistinctness 

 of an image from the unequal refraction of the rays of light passing 

 through the circumference and the center of the lens ; it is corrected 

 mainly by the iris, which cuts off the marginal rays, and transmits 

 only those passing through the center. 



Chromatic aberration is a condition in which the image is sur- 

 rounded by a colored margin, from the decomposition of the rays 

 of light into their elementary parts. 



Myopia, or shortsightness, is caused by an abnormal increase in 

 the anteroposterior diameter of the eyeball, or by a hypernormal 

 refracting power of the lens. It is generally due to the first cause; 



