Chap. XX] INTERNAL AND EXTERNAL SENSES 431 



aqueous humor, the vitreous humor, and the crystalHne lens which 

 have just been described. 



Accommodation. — Accommodation is the ability of the eye 

 to adjust itself so that it can see objects at varying distances. Tlie 

 changes which occur in the eye during accommodation for near 

 objects are three in number : (1) the pupil contracts, (2) the 

 lens becomes more convex, and (3) the axes of the eyeballs are 

 turned inward by the action of the internal recti muscles. 



A normal eye is capable of distinct vision throughout an im- 

 mense range. We can see the stars millions of miles away, and 

 with the same eye, though not at the same time, we can see ob- 

 jects within a few inches of us. To be able to see objects mill- 

 ions of miles away and within a short range, the eye has to 

 accommodate or adjust itself to different distances. This accom- 

 modation is accomplished mainly by the lens changing its con- 

 vexity. In accommodation for near objects, the lens becomes 

 more convex (advances), and the pupil of the eye hkewise con- 

 tracts. This convexity is brought about by the action of the 

 ciliary muscle, and is always more or less fatiguing. The ac- 

 commodation for distant objects is a passive condition, the 

 convexity of the lens being unaltered and the pupil of the eye 

 dilated, and it is on this account that the eye rests for an indefi- 

 nite time upon remote objects without fatigue. 



Common conditions that affect accommodation. — The condi- 

 tions that affect accommodation are : (1) hypermetropia, (2) myo- 

 pia, (3) presbyopia, and (4) astigmatism. 



H3rpermetropia. — Hypermetropia or far-sightedness is a condi- 

 tion in which rays of light from near objects do not converge soon 

 enough and are brought to a focus behind the retina. This is usu- 

 ally caused by a flattened condition of the lens or cornea, or an 

 eyeball that is too shallow, and convex glasses are used to con- 

 centrate and focus the rays more quickly. 



Myopia. — Myopia or near-sightedness is a condition in which 

 rays of light converge too soon, and are brought to a focus before 

 reaching the retina. This is the opposite of hj-permetropia and 

 is caused by a cornea or lens that is too convex, or an eyeball of 

 too great depth. To remedy this condition concave glasses are 

 worn to disperse the rays and prevent their being focussed too 

 soon. 



