500 



THE HUMAN BODY. 



Accommodation is brought about mainly by the ciliary 

 muscle. 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 according to the degree of its contraction 

 slackens the suspensory ligament (p. 491), and then the- 

 elastic lens, relieved from the lateral drag, bulges out a 

 little in the centre. 



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

 accommodation 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 re- 

 tina. In the normal eye par- 

 allel rays meet on the retina 

 when the ciliary muscle is 

 completely relaxed (A, Fig. 

 133). Such eyes are emme- 

 tropic. In other eyes the eye- 

 ball is too long from before 

 back; in the resting state par- 

 allel rays meet in front of the 

 retina ( B}. Persons with such 

 eyes, therefore, cannot see distant objects distinctly without 

 the aid of diverging (concave) spectacles; they are short- 

 sighted or myopic. Or the eyeball may be too short from 

 before back; then, in the resting state, parallel 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 (con- 

 vex) spectacles. Such eyes are called hypermetropic, or in 

 common language long-sighted. 



FIG. 133. Diagram illustrating the 

 path of parallel ravs after entering 

 an emmetropic (A), a myopic (J?), 

 and a hypermetropic (C) eye. 



