226 THE HUMAN BODY 



curvature is 10 mm. (f inch), when a very near object about 

 6 mm. (-^5 inch), and this change is sufficient to account for the 

 range of accommodation of the normal eye. 



When the eye is focussed for seeing a near object the circular 

 muscle of the iris contracts, narrowing the pupil, but this has 

 nothing directly to do with the accommodation. 



Accommodation is brought about mainly by the ciliary muscle 

 (Fig. 89). In the resting eye it is relaxed and the suspensory 

 ligament of the lens is taut, and, pulling on its edge, drags it out 

 laterally a little and flattens its surfaces, especially the anterior, 

 since the ligament is attached a little in front of the edge. To 

 see a nearer object the ciliary muscle is contracted, and accord- 

 ing to the degree of its contraction slackens the suspensory liga- 

 ment, and then the elastic lens, relieved from the lateral drag, 

 bulges out a little in the center. 



Short Sight and Long Sight. In the eye the range of accommo- 

 dation is very great, allowing the rays from points infinitely distant 



up to those from points about eight 

 inches in front of the eye to be brought 

 to a focus on the retina. In the normal 

 eye parallel rays meet on the retina 

 when the ciliary muscle is completely 



^^-- **^ relaxed (A, Fig. 90). Such eyes are em- 

 metropic. In other eyes the eyeball is 

 too long from before back ; in the resting 

 state parallel rays meet in front of the 

 retina (B), Persons with such eyes, 



FIG. 90. Diagram iiiustrat- therefore, cannot see distant objects dis- 

 ing the path of parallel rays tinctly without the aid of diverging 



after entering an emmetropic 



(4), a myopic (B), and a hy- (concave) spectacles; they are short- 



permetropic (C) eye. -7.7 /-\ , i in 



sighted or myopic. Or the eyeball may 



be too short from before back; then, in the resting state, par- 

 allel rays are brought to a focus behind the retina (C) . To see 

 even infinitely distant objects, such persons must therefore use 

 their accommodating apparatus to increase the converging power 

 of the lens; and when objects are near they cannot, with the 

 greatest effort, bring the divergent rays proceeding from them 

 to a focus soon enough. To get distinct retinal images of 

 near objects they therefore need converging (convex) spectacles. 



