SIGHT 229 



With a double convex lens the image formed is real, inverted, 

 and on the opposite side of the lens from the object. The 

 crystalline lens is a double convex lens, and obeys the laws just 

 described for other lenses. In addition, there are other refract- 

 ing media in the eye the cornea, the aqueous, and the vitreous 

 humors. The crystalline lens is, however, the most important, 

 as it possesses the power by virtue of the ciliary muscles of 

 increasing or diminishing its curvature. 



By accommodation is meant the power of changing the amount 

 of curvature of the crystalline lens so as to throw the image of 

 an object in exact focus on the retina whether the object be 

 near or far from the lens. At the same time the pupil is expanded 

 or contracted to admit the necessary amount of light. Thus, 

 if an object be near the eye, in order to produce a sharp image 



FIG. 14 



Formation of image by convex lens. 



the lens is more curved, owing to the contraction of the ciliary 

 muscle, and the pupil is contracted. If, on the other hand, the 

 object be on the horizon, the ciliary muscle relaxes, the lens is 

 flatter, and the pupil is dilated. Accommodation is an example 

 of a voluntary act brought about by the action of the unstriped 

 fibers of the ciliary muscle. The fact that most people must be 

 assisted by visual sensations does not alter the fact that it is 

 the result of the will. The nervous path of accommodation is 

 through the anterior part of the nucleus of the third nerve in 

 the floor in the third ventricle, the anterior bundle of the nerve 

 root, the third nerve, the lenticular ganglion, and the short 

 ciliary nerves. 



Atropine paralyzes and physostigmine stimulates the ciliary 

 muscle. Associated with accommodation for near vision there 

 is, besides the contraction of the pupil, a convergence of the 



