THE ORGANS OF SENSE. 



125 



the eye moves backwards, showing that the point in the ciliary processes 

 has been pulled forward. 



When the ciliary muscle contracts in accommodation, there occur 

 at the same time contraction of the pupil and convergence of the eyes. 

 The contraction of the pupil is effected by the sphincter pupillae, and 

 is of service in sharpening the definition of the image formed on the 

 retina, just as the definition of an image in the photographic camera 

 is improved by the use of a small diaphragm. The convergence of 

 the eyes is effected by the contraction of the internal recti, and results 

 in the image of the object looked at being formed on the fovea of 

 each eye. 



In the emmetropic eye the far point (punctum remotum) of distinct 

 vision is at infinite distance, while the near point (punctum proximum) 

 varies with age. The elasticity of the lens, and consequently the range 

 of accommodation, diminish steadily as age advances, and the near point 

 therefore gradually recedes. This is shown in the following table : 



In civilised life the power of accommodation is called into play more 

 for reading than for any other purpose, and it will be seen from the 

 table that between the ages of forty and fifty the near point recedes to 

 a greater distance than it is convenient to hold a book. Moreover, it is 

 found that the prolonged effort of accommodation required for reading 

 cannot be kept up if more than three-fourths of the total power of 

 accommodation is being utilised. It is therefore necessary, usually about 

 the age of forty-five, to supplement the mechanism of accommodation for 

 reading or other near work by the use of convex lenses of such strength 

 as to bring the near point to a range of about 25 cm. or ten inches. 

 The term presbyopia is used to indicate the failure of accommodation 

 which occurs about the age of forty-five. 



Accommodation is a voluntary act, and is peculiar in that respect 

 in that the ciliary muscle and the Sphincter pupillse are composed of 

 smooth muscle. Both these muscles, as well as the internal recti, are 



